In Hellenic religion we have so many water entities and divinities that is actually quite fun in and of itself. Perhaps it is because the sea is regarded as the in-between splace, whether that be between the heavens and earth, or the living and the dead. As its fluid nature is perfect to represent this liminal spiritual space.
Olympians in the sea
Of the Olympians the god of the sea is quite obviously Poseidon, who also has many connections to fresh water sources such as springs, and has some connections to the rains as the rainy season really kicks off in the winter month that bears his name and the festival of the month. Of course that this is interconnected big time with Zeus who is likewise associated with the rains, perhaps more prominantly, as is his wife Hera who is like the clouds that become engorged with the rains and holds the title Telchinia which associates her as a storm goddess, it can certainly infer a relationship between Zeus and Poseidon in which the Orphics called the Three Zeuses (Haides included among them). This relationship between Zeus and Poseidon perhaps can be understood better through the relationship of their cosmic predecessors, Okeanos and Ouranos. This is because Okeanos, the sea of the world which encircles our world spiritually/mythically, and father of all rivers, rainy vapors etc, is a reflection largely of Ouranos which is the aetheric body that surrounds all things, the so-called heavens or skies. Therefore there is very little that distinquishes the heavens from the seas, and both are described almost in a liquid form, and the latter in nature reflects the former.
There are of course other Olympians who have sea associations. Apollon is a rather big one, who, like Poseidon, is associated and depicted with dolphins. However the part of the dolphin between them may be a bit different. Dolphins are perhaps among the most intelligent creatures of the sea, not to mention possessing a cooperative nature that would make them valuable. With Poseidon this may be linked more with Poseidon’s movement through the seas. With Apollon though it is perhaps more specifically that the dolphin is associated with the upper strata of water that is oxygen-rich from various plankton and seaweeds that grow close to the surface where they can reach the light and the nourishing rays of Apollon. Moreover in connection with Apollon we have the dolphin presented as the guide, particularly in the homeric hymn to Apollon in which the god takes the form of a dolphin to quite literally lead the Cretan Sailors to Delphi. Thus Apollon as the dolphin is the guide of the soul-boat of men. This soul boat of men is best reflected in the imagery and myth of Dionysos and his vine-covered ship (the ex-pirate ship) the pirates of which were turned into dolphins that seem to perform a protective circumference around the vessel. Seasonally this also contrasts to Poseidon who is associated with the winter rains, and a largely dangerous time to be at seas which sensible sailors avoided, Apollon’s dolphin rises the heavens as a constellation which marks the beginning of the spring season, and the return of the god, and favorable weather returning for sailors.
Artemis is also connected with water to a great extant as a goddess who delights in rivers and springs, and had a number of them throughout Hellas that were sacred to her. She also seemed to be identified with the nursing river of Zeus in a manner not unlike the Nereid Eurynome who was the nurse of Hephaistos for which she was called Artemis Eurynome in relation to this function, and as a strong identification of her aquatic nature was depicted with the tail of a fish like a mermaid. Artemis is incidentally one of those goddesses who are called Soteira, specifically in association with her being a savior of sailors. This perhaps not surprising for a goddess that is in mystic traditions called the daughter of Demeter and Poseidon. It is quite likely that the cults of Apollon and Artemis as sea-associated deities has a great deal to do with their relationship with Zeus and Dionysos on Samothrake, part of the benefit of which included the safety at seas for initiates.
Athena meanwhile is strongly connected to Triton, the river near which she is said to have been raised in myth. Boeotia claimed this river to be theirs, as to do the people of Crete, and likewise in Libya where a lake takes that name. In the latter case the Libyans claimed that Athena was a daughter of Poseidon who was adopted by Zeus because of her negative relationship with her father.
Then there is Aphrodite, the seaborn goddess daughter of both Ouranos and Zeus, who seems to be associated particularly with fertile nature of water, its beauty and its natural mirror-like tendency when calm. Like Artemis she too has certain springs and fountains that are sacred to her, and like Artemis is often depicted as a bathing goddess. Though Hera too is said to have bathed to restore herself.
River gods and Minor sea gods are literally the spiritual presence associated with specific bodies of water. Usually this is particularly the case with major rivers that often also have a cult significance in relationship with other gods. Some nymphs can also be placed in the spectrum of river gods in cases which a nymph is tied directly to body of a specific river, as is the cases of one of the nurses of the infant Zeus in Arcadia. Usually though river gods are conceived as masculine father gods with nymph daughters. Such is the case of the Ladon who have several daughters that figure prominently in myth, one example being Daphne in the Peloponnesian version of her myth, whereas in Thessaly and elsewhere nearby she called the daughter of the river Peneios. River gods seem to have been considered pretty powerful divine beings, and as such we not only see Peneios offering to shelter Leto from the wrath of Hera to give birth (though Leto refused so that Peneios would not suffer the consequence) but also in perhaps the greatest known river god, Xanthus who not only had stature for being the place where the infant Apollon was bathed and the river dedicated to the son of Leto, but also figured prominently in the Iliad as one of the gods defending Troy (though which is appropriate as Xanthus was in the lands of Illium. Therefore we can see a great relationship too between the river god and civilization, perhaps so because civilizations naturally formed near rivers and thus were probably viewed not only as potentially protective and puritive bodies, but also have paternal (or maternal as the case may be) characteristics that brought fertility to the land around. River gods typically represented as divine beings with two fish tails in the place of legs, often blowing horns made of shell.
It is also quite possible that some of the divine sons of Poseidon, such as Triton, were considered something akin to river gods, though perhaps associated more specifically with currents that may have been viewed as underwater rivers that created the movement of the sea waves. Triton is indeed called the herald of his father Poseidon and is credited with the ability to calm waves with his conch-shell horn. I consider him plausibly as a kind of under-sea river god because like river gods he is usually depicted similarly with the two-tails.
Nymphs, as I said above, where often ascribed as daughters of rivers. This seems particular the case of Naiads, fresh water nymphs that inhabited around streams, rivers and lakes. So called sweet-water nymphs. There are, of course another classification of water nymphs, Nereids, fifty in number, the daughters of Nereus the old man of the sea (likely a predecessor of Poseidon whose is represented with the lower half of his body like that of a fish). Some of these Nereids are quite famous, such as Thetis, the mother of Achilles. Whereas Naiads are often featured strongly in association with nurses and seduction of shepherds who wander nearby their vales, Nereids, while also taking the form of divine nurses, are also associated with purification, as we know that the initiates bathed in the seas after having descended from the temple of Apollon Daphnaeus and that the Nereids are called those who were the first to witness the mysteries (quite probably again due to their role as divine nurses) for which they are presented as being present as a chorus as the initiates travel to Eleusis.
It is of course interesting to note that water nymphs have been presented as attending to and adoring Apollon, as well as a number of them being among the nymph companions of Artemis.
This is perhaps one of my favorite water entities. The hippocamps were creatures that were half horse with the tails of fish for their hindquarters. These served as mounts for sea gods, nereids and the like, and are often depicted pulling the chariot of Poseidon. I have also personally imagined the foam crests of waves to be like the manes of these horses as they ride through the waves, carrying the Nereids in their play. Of course there are other fish tailed creatures that seem to be act as mounts for other gods, such as pardolokampos that is half leopard that appears in art as a mount of Dionysos and in another case it is Astypalaia (in which case the leopard fish is actually the form taken by Poseidon..though in this case the pardolokampos is also winged), and the Aigikampos on which Eros has been depicted riding. The Bull form Zeus took to carry of Europa is also depicted with the hindquarters of a fish. Triton meanwhile is carried by a leokampos which is half lion. However, it is likely that these representations may have all originated from the concept of the hippocamp as mounts of the seagods, as these are vehicles by which the gods are traveling over the seas.