I'll be perfectly honest with you. On this trip just enjoy the snorkeling. If you come away from the trip thinking snorkeling is just not enough, get certified or certification ready for your next trip. As Subsidy said, diving is addictive and an expensive hobby. My whole life is different because I became a diver, different good but life has become more expensive than I'd originally expected it to be.

Not that I'd change a thing . . .unless it was to dive even more!
Anyway, your thinking that it might not be the best thing to do on THIS trip, is probably the right approach this time. Have a wonderful time! I know you will.