[Septimius Severus]

I will say that I think the majority of R'leyh troops both recruitable and freespawn are amphibious but yes as rdonj and chris point out they have a few that are aquatic only, which could be fixed. Of course, the R'lyeh player could opt to play them with a high growth/high order/low dom which might not be so bad given the water area plannned (at least in the water) and given they'll have no direct competition in the water. But of course this is not necessarily the best way to play them, and certainly a high death, high dom is also a very good option. Up to the player.

LA Atlantis is also an option. All of their units are amphibious and they don't have to worry bout freespawn. The only thing with them is their start would have to be a land start. Which would tend to necessitate the need for several provinces of land in the midst of this central sea. Of course I could make the center water province a land province only and just give a resource bonus site, but they'd not be able to recruit Tungaliks en masse until they took some land. Forgiving fathers have a little more research but don't share all of the same paths and are more expensive. Mournfuls would also not be able to be recruited outside of the cap until a land province was taken. Though Arsaturts (cap only) and Unsleeping/Unsleeping consorts should be plentiful and are probably the most powerful recruitables LA Atlantis has.

Of course if people prefer a land nation for the indy nation I am throwing in, that is of course very possible (wouldn't require water either). Only thing is they'll be surrounded by the AI caps and might be more vulnerable. If the AI should end up being defensive in nature, like the current game, Atlantis or R'lyeh could pretty much attack without fear of losing these core holdings. But Ermor or another land nation would have to defend itself and might be more hard pressed.