[Lazy_Perfectionist]

Well... I'm not going to be involved, but if you'd like my suggestion for middle era themes-teams based on flavor...
I really don't think the water teams belong together. They have a long antagonistic relationship to each other. Unfortunately, my suggestions might not mesh well with random. Feel free to disregard, since I'm not involved in the game. Are you going with themed teams, or merely themed random factions with teams not necessarily of the same theme?

What I'd do is split them up, and attach them to teams without any amphibious presence.

Agartha, Shinuyama, Ermor have some level of amphibious presence. I can't remember off hand who also has easy access through nationals or summons, but you get the idea.

As a team, Agartha, Jotunheim, C'tis, and Caelum are very uncomfortable allies - two coldblooded nations and two cold-thriving nations? Rules out some global enchantments, makes border dominion uncomfortable.

All that said, a team of watery factions makes good enough sense if you ignore the fluff history of the races.