Some of my favorite creatures to encounter while snorkeling are the porcupine fish and the balloon fish (both in the Diodon genus). Of course, we have to try to puff them up. I know this is aggravating to the fish... but it usually does not take much. Just poking them gently (wearing gloves!) or trying to grab hold of them will cause them to puff up. Some are more difficult to go after than others, though, and are able to out-swim us. It's usually the little ones we can get our hands on. We also limit our time with the fish, and let them deflate/swim away after a few quick photos.
For those that know me, getting hands on with nature is a must... I don't understand how anyone can go snorkeling without gloves on.