[Endoperez]

LA Mictlan and EA Agartha also have underwater defense. MA and LA Agartha have PD units that don't survive underwater, but their entry on the manual claims it includes underwater defense. You can't recruit PD, so it's a typo in the manual. MA Agartha might be intended to have some kind of PD underwater.

Kelp Fortresses can be found from Nature sites. They were very common in Dominions II.

Because we do have commands for modding what forts different nations can build, it would be pretty simple to give all landlubbers a single fort they can build underwater. It would be done with #selectnation <nation nbr> and #uwfort <fort nbr> commands.