OK, lets walk through this....
You're on the surface, face down, snorkel clippped to the left side of your mask, your breathing....... You spot something on the bottom and decide to surface dive to check it out. So, you would take a breath of air, holding it in your lungs and tuck and dive.
Acsend, hand up (making sure you don't bonk your head on anything) and then level out flipping your head forward and expelling the last breath of air from your lungs, clearing the snorkel as you get to the surface. Sounds simple enough eh?
Karen, I'm thinking you have a
WalMart special that leaks either at the swivel or at the mouthpiece. There is a little round piece of rubber on the bottom of the snorkel that acts as a valve. If this gets cruddy, folded or damaged then water will come in when it's not supposed to! Check it out or have Mike do some exploratory surgery and see if that is the problem. You might want to practice at the pool prior to the cruise to ensure proper operation.