I'd actually guess it's more of a conceptual thing than implementation. It's relatively easy to come up with concepts for new terrestrial nations. Underwater, though? Especially with all the ground the existing three cover?
Also, I'd say that two immediate problems come to mind with the above-proposed undersea free-for-all. First, it's going to be ugly and annoying in that it'd be flatlands with almost no chokepoints. Second, neither EA Agartha nor LA Ermor would exactly be enjoying themselves - Agatha could at least build PD and forts, but their underwater troops are awful enough that they'd effectively become capital only as far as everything was concerned. Ermor would be denied fortresses (which are extravagances, yes, but needful ones) and the token one point of PD to avoid having their skellie farms be scout-killable. LA Atlantis would fare better than these two, but they'd still be significantly weakened.
It's an interesting idea, though. If you really wanted to have amphibious terrestrials, they'd probably each need an island with 3-4 provinces to make it fair, and possibly have isolated island provinces scattered about. Heh. If you did that, you could actually add LA Mictlan, though that'd be very, very painful - the only amphibious troops and commanders are capital-only sacreds, and they can't build forts underwater even if they do have PD. To say nothing of how vulnerable their handful of blood-for-domain harvesting operations would be.