I've had good luck with fresh scallops (at least for a while, until my bimac grew tired of them). Now he likes good quality frozen shrimp (the kind you buy in the fishmarket). I've found that the largest ones are harder for him to digest, so I buy medium (16-20 to the pound).
My new baby, Lil Pumpkin, (my daughtor named him) has been eating small hermit crabs and the local shrimp we can catch. Ink used to eat crabs, fish, snails, frozen prawn, just about anything I put in there.