[Taqwus]

Some undead will get you underwater (not the fleshy ghoul / wight types, nor vampires)... but you should beware R'lyeh while doing so, because if you have mindless hordes led by a few Revenants or Mound Kings, all the mindblasters will focus on the leaders.

Mechanical Men and Living Statues also are amphib; same caution applies as they're mindless. You'd want to use your astral mages to forge some antimagic items, and ditto earth if you've got a non-spellcaster like a Mound King leading troops (Lead Shield + Antimagic talisman ain't bad; Cons 6 gets you Starshine Skullcap for another +2 MR).

You don't have a pretender with heavy Astral, do you? Astral 5 gets you Imprint Souls, IIRC, which works underwater -- although the priests it summons will also be insane. Of course, you could also cast it on the main continent.

Atlantis is not only sailing but amphibious in Late Era with a lot of their troops. No mindblasters, 'tho, and at least their dominion won't drive your commanders insane, nor will they have Vastnesses.

I see quite a few forests... might be a chance of an Enchantress site.