Why does my hermit crab keep coming OUT of his shell and sitting in his water bowl?

because the environment is off ... there is something in his shell and no spare shells to change into or maybe the tank has mites ... and if you have ONE hermit crab, that is about as long as they live ... if kept in a proper environment they live over twenty years !!! rescue pet, i guess you did not do such a good job in the rescue department if the crab is dieing ... because healthy crabs do NOT come out of their shells ...

How long has he been doing this? He could be trying to molt, but is stressing if the environment is not appropriate for him.

Or, as others have mentioned his shell may be too small, but I can help you out to tell if this is the issue.

http://www.usshell.com/ProductImages/hermitcrabs/c... When he pulls into his shell, does he hang out like these two? Both crabs are in shells that are far too small for them. It may not be the owner's fault though, I did have one who was very stubborn, and would refuse to leave his small shell that even had a hole in it.

They need to be able to pull in completely like this: http://3.bp.blogspot.com/-6RxKmPCiU8Q/TV6y-I67_KI/...

Also, do you have a bowl of fresh, and salt water available? They need both. This could be the issue. I found that my crabs always liked to clean out their shells in the salt water bowl. Though they spent more time drinking in the fresh.

He might be outgrowing his shell. Does he have larger shells he can try out?

Also a hermit crab that is naked may be under some sort of stress.

http://exoticpets.about.com/od/hermitcrabs/f/naked...

That site has some ideas as to why you might have a naked crab and what you can try to get it back into a shell.

He needs a bigger shell and fast!