Serpents and Kraken do work well together, but that is 1475 gold and a couple of cities devoted to units that can only attack a limited number of cities. May make sense on a few maps, but probably not most. I'd like to see a one turn aquatic unit (maybe merfolk?), make them slower than the others(12-16 speed), 10 strength and a water bonus.
Ezras, I'll check out this Waterworld of yours. Naming my first hero 'Costner' for sure.