Mr_S is right, snorkelling is a completely different ballgame. Just try some 'experience' dives. PADI do 'try' dives that give you the experience of diving without having to do the course (I'm sure many resorts do it as well with varying degree's of legality). This will give you the experience of being able to look up and see the surface whilst being weightless. I love that feeling - it's fantastic. But then I grew up 100m away from one of the best surf beaches in the world ...
My tip for you would be to remember to breath slowly and deeply. Not only does it conserve your oxygen but it will calm you as well.
Give it a go, you'll be surprised at how easy it all is and how great it can be to be down there with the fish. You'll be doing your advanced open water in no time and sitting on the bottom at 28m tying a knot in a rope to prove you're not narc'd!
