[Micah]

Most land nations cannot purchase PD or construct forts in the water, but there are a couple of options for getting one. The cheap but unreliable method is searching for N sites, as one of the magic sites spawns a free fortress. The expensive option is to cast a spell (Living Fortress I believe, but don't quote me on that.) It's something like 40N to cast though, plus you have to have the spell researched, and I think you might have to have your mage underwater to cast it to boot.

Forts don't tend to interact with rituals unless the rituals attack the province, like Ghost Riders.

Underwater forts for land nations are pretty much only good for defence. Resources in the water suck, UW fort admin values suck, and the recruitables in the water also pretty much suck, and of course you can't recruit any national units down there as a land nation. That being said, there are some rituals that can only be cast in an underwater lab, so having one fort in the water is often handy. (A lot of remote spells can't be cast on an UW province, but that's pretty niche.)

Hope that helps.