Seems way overpowered. I'd use that spell to strip all the items off any enemy SC's, and to strip large armies of their Wine bags, leaving them starving. Even if the cost was made more in line with its power, stealing clams would be pretty low, priority-wise, because you would't be able to cast it enough to make a difference..
Also, the clam hoarder is likely far better equipped to spam this spell than his opponents would be, since he'd already have a good water gem income, and a bunch of water mages.
I'm not really an opponent of clams (within reason), but if you want a water-based ritual spell to combat clams, how about
:
Salmonella- W7, 80 water gems
- Global effect: Every clam holder has a 30% chance per clam per turn of getting diseased. 