Which marine pairings typically exhibits mutualism?

Barnacles/mussels

Clownfish/anemone

Shark/sea lion

Tapeworm/manatee

I believe the answer is: clownfish/anemone

Clownfish is very territorial in nature. So, when they hide within the anemone, the clownfish would prevent smaller fishes from eating the anemone. In return, the anemone would prevent the bigger fish from reaching the clownfish when they hide in it.