I've had a barracuda take a bit into my fin and it didn't let go for quite a while. Sure was glad it wasn't any part of me. I never wear jewelry when diving or snorkelling, not even a ring.
Of course, there are other reasons not to wear rings. I'm sure most of us have heard the true story of the guy who jumped off of a boat and caught his wedding ring on something as he went into the water . . .degloved his finger. Yikes.