Shouldn't it stand to reason that if they're based on water, they're weak to lightning?
I'm in the Sea Serpent Grotto taking these out, trying to get the chest key.
I kept track of my casting results. It's Full Moon, Windsday.
Thunder I spell - averaging 113 pts of damage
Fire I spell - averaging 79 pts of damage
Seems weaker to lightning-based spells than fire.