Water III: Tsunami.
Although Tsunami fits with the octopus overlords theme, it's pretty disappointing that low level priests have more command over the power of water than an archmage with Water III. It's also frustrating that if you seriously want to fight at sea, you're pretty much railroaded into OO, and water mana is almost irrelevant. I don't think the ability would need any change, but just make the exact same spell that cultists have, available to archmages too.