My wife spotted this crab scuttling quite rapidly along the floor of the bay where we were snorkeling. It was clearly some kind of hermit crab, but it was quite deep and this was the best photo I got.
When we got home, I looked through my copy of John P. Hoover’s Hawaii Sea Creatures and the jeweled anemone crab seemed the most likely identity. As can be seen in the photo, the shell is covered with anemones which is a feature of these crabs. Since these crabs are mostly night feeders, we were lucky to see one still active in the early morning. Possibly it had been disturbed and was headed to a new hiding place.