[Cohen]

Personally I think the Clam Hoarding skill of Atlantis supply totally to the cost of his mages.

Indeed the total range of Magic that is affordable by Atlantis is great.
Underwater Voice of Tiamat rules. You start to gain some income for casting all other site searching spell on surface except death and blood ... but you can always alchemize your astral pearls (clam clam).

About combat magic, Atlantis has some more difficulties, but the fact every King of the Deep can quicken, and start firing spells of every element for the Atlantis enemy is difficult to say from what to protect.

In addition Atlantis (and Ryleh) has a lot of Globals and RItuals suited for his defence.