LOL, if you haven't even snorkelled first, well, heck. Why not snorkel this trip and get certified at home for your next trip.
To check out your aptitude for diving you should be able to get into a Discover Scuba in the pool at home. It's usually only about $25.
I'm telling ya, you are going on holidays to a really fun place and facing a big exam when you get home. Do you really want to be closely monitoring your alcohol consumption and what time you are getting to bed? I didn't think so.