Hmmm...telling the kids...
Plan 1 was not to say anything at all...until arriving at the ship. But having been to southern Florida before, the kids would see through that plan as soon as the pilot mentioned the word Florida, especially since they expect to go skiing in Banff. Plan 2 involved a secret Mickey postcard coming our way from Verandah Man (see the postcard thread - though it may be too late for your little one). We had planned to hide the postcard, plant it back in the mailbox two days prior to departure, and then let the kids get the mail that day. But the logistics of packing have made us revise that plan. Revised plan 2 is to let nature take its course and let the kids get the mail every day, knowing that at some point, the card will be there. Then we'll watch the
Disney cruise video together so they get a good feeling of what's ahead.
Doing the packing in secret is just too difficult. We both work and we ski each and every weekend, so the only time left to pack is well...never. Once they know, I'll put the suitcases out and do it bit by bit.
Now for your snorkeling/diving question. Nick and Josh were first introduced to the water at the ages of 3 months and 7 weeks, respectively, at a special baby swim school here in Montreal, so it seems they've always known how to swim. Nick could get from the center of the pool to the edge before he could walk well. Pool snorkeling followed a few years later, but saltwater snorkelling is relatively recent. It's the taste factor that kids have a hard time overcoming. But once they see a fish of any kind, the water suddenly seems to taste better

and then they're off and running. As for scuba diving, my husband and I both used to but gave it up with the arrival of the kids...simply not enough time. Once Josh is 12 and Nick 14, we'll put them in PADI. By then, I'll have to redo my open water anyway, so I'll probably redo it with them. My husband has a higher certification than me...but is probably just as rusty as I am at this point! I envy Christine in that they seem to have kept it up in spite of the kids...
If your little one is comfortable in the water, get him an inexpensive mask and snorkel for experimenting in the pool on the cruise (hmm..is that allowed on the boat...I don't actually know). He won't like the mask at first and may spend more time just breathing through the snorkel than anything else, but with practice he'll figure it out. Our neighbors' youngest is 4 and he is a regular visitor to our pool. I keep one of the boys' starter mask/snorkel sets around and Matthew does his best to "use" it like the big kids.
Hope that helps...
BTW, we're also on Deck 6 and have second seating...