Prior reviews have covered in detail what to expect - see BLM-AK's review for some great detail. No, you aren't snorkeling - the mask/snorkel are simply to make it easier to keep your face in the water so you don't miss a thing. And yes, the shower is hot (one review I read said it was warm - not so!), and water/pineapple/home baked cookies are all provided. Only thing I would add is check the weather report - we purposely scheduled our trip on a day the winds had died down so the sea would be calmer (I tend to get sea sick). I found crossing my ankles helped to keep the foam noodle in place.
On our trip our crew members were Shauna and Victoria. They were really efficient and communicative about what to expect and super helpful once we were hanging on to the lighted boards - making sure masks were all fitting correctly, keeping "noodles in place", alerting us to oncoming manta rays. When you first get in the water it doesn't feel that cold, but you are hanging on to the board, stationary, for over 30 minutes so it can get chilly. On our way out we saw 2 whales - within 50 yards of the boat - so Captain Bill slowed down so we could watch them. Once we tied up other boats arrived, and some of them got their lighted boards in the water before the sun set, and therefore drew most of the manta rays away from us. We waited and waited and only saw about 3 rays. Captain Bill decided to hang on until AFTER the other boats pulled their boards out and OH MY GOSH - we had at least 6 rays under our board at one time doing their belly rolls to eat the plankton. It was AMAZING!! They are huge - 10 - 14'?? and come within inches of you. Some people actually got brushed as they swam by. We were lucky our boat wasn't satisfied with only 3 rays and waited to provide the best possible experience. The whales were icing on the cake.