[HoneyBadger]

I would think that MA Agartha-with the inclusion of humans and all our bright shiny ideas-would have some troops specifically tailored to fight water empires. True, they still might not hold up as well, might be costly in terms of gold/resources, but they ought to have some soldier types which are there incase the player wants to spend a lot of gold/resources and tackle water. A national summons would work here too. They shouldn't have the ability to go underwater unless they have some kind of strategy to back it up, and I think that once the humans showed up, they'd start exploiting that in some way.