There's a goblet thingee that you can forge for him (or that he can forge) that will allow any commander to bring 50 human-sized troops underwater with him, or 25 giant-sized. (Minus one for the commander, though)
Undead work, too, though at a disadvantage (Poor Amphibians). Soulless in particular end up with Attack/Defense/Etc. in the 2-4 range. Spectrals work well if you're LA Ermor.
Use the sea-troll to craft a Sea Goblet, next turn equip it, and you can bring a nice number of units under the waves (plus the trolls who were summoned along with the king).
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