The Dark Sun Campaign Setting book has the Rainbringer. That's the only paragon path I could find that specifically focuses on water.

There are some more that are vaguely water-related.
  • Elemental Savant, from Heroes of the Elemental Chaos, has water options.
  • Divine Power has the Astral Savant, which has powers that draw on the Astral Sea.
  • There's also the Sword Coast Corsair in the Forgotten Realm's Player's Guide. More of a pirate than a mage of elemental water, I'm afraid.