Well, I pretty much did what I wrote in MP, but have to admit my only battles were 2-on-1 versus EA Ermor, not the hardest of challenges. And I got the 50% water enchantment discount site and used it to pump out my claymen. But I did win all the land battles I fought, mostly medium-sized and including enemy mages, and I think I captured at least a couple castles. (I had to sub out of the game because of travel).
I site searched manually with H, W2N2, W4E initially to get good early gem income, and cast VoT mostly in deep water. I also killed Atlantis and R'lyeh, so I had their income and the income from the provinces they had searched.
You're right that Sea Trolls don't do a whole lot of rampaging. (Actually, Quickened Sea Trolls with Weapons of Sharpness really do rampage. That's a bit later on in the game though). But I think they are a pretty resilient meat shield for your mages to hide behind and either spam poison (with Foul Vapors up) or acid, and they give enough cover for the Sea King himself to race around and kill stuff out without getting killed himself. And hey, the King pays back all your gems after 50 turns.
I've got turn 28 of a different underwater MP game saved, and looking at my income, it's: +4F +2A +25W +14E +9S +4D +15N. I think that's pretty typical of the mid-game after defeating 2 other water nations on a full map.