I think it should be either nagas or water spirits, but definitely not humans, elves, etc.
1) We already have elves in the army.
2) Nagas already have sorcerers.
We could introduce a creature called "Tide Guardian" or something similar at level 6. However, this idea is not easily implemented. A humanoid creature with tentacles and wings :D
There's also the idea of a giant sea turtle... but that's also not easy to implement :mad:
However, these Tide Guardians could be implemented as a ghostly embodiment... we could even use a model of the undead... that's all that comes to mind... not counting giant crabs, but that's unlikely to be feasible at all :confused: