Blessing of Poseidon

you gain a swim speed equal to your movement speed and can breath underwater. Fighting underwater gives you no disadvantage.

You also get access to the ability to cast certain spells at certain levels using the mental stat of their choosing for their spell casting modifier. You can cast each only once per long rest.

3: Find Steed (always a horse that can run on water)

5: Water Breathing

7: Control Water

9: Summon Elemental (water only)