Why is it a big deal for Marcellus to retrieve the class ring from the wolf eels' tank?

Marcellus decides he has to retrieve the class ring from the wolf eels' tank. It is a big deal because before he was captured and imprisoned, he was attacked by a wolf eel and lost one of his arms. That was when he was captured and imprisoned. He knows how dangerous wolf eels can be, but it is so important to get the ring that he takes the chance.