There wasn't a specific Greek god *solely* dedicated to swimming.
However, the god most closely associated with the sea and therefore with swimming would be Poseidon, the god of the sea, earthquakes, storms, and horses.
Other figures connected to water and swimming in Greek mythology include:
* Oceanus: The Titan god of the great river that encircled the earth.
* Nereus: An old man of the sea, known for his wisdom and truthfulness, and father of the Nereids.
* The Nereids: Sea nymphs, daughters of Nereus, who were known for their beauty and helpfulness to sailors. Thetis, mother of Achilles, was a Nereid.
* Triton: A merman messenger of the sea, son of Poseidon and Amphitrite.
* Amphitrite: Poseidon's wife and queen of the sea.
So, while none of these are exclusively gods of swimming, Poseidon's dominion over the sea makes him the most relevant deity in that context.