On all the cruise ships my DH and I have been on, they give you a short course when you get to the snorkel area and get your equipment. If you're renting just the snorkel equipment for the day, they have (at least on other cruise lines) a short course when you rent the equipment.
I have contact lenses and have never had a problem snorkeling with them. Just make sure your mask is air tight. They show you how to do this. I have gotten water under my mask a couple of times but find that it isn't that bad, at least, not any worse than witout contacts. I always bring an extra pair but I've never lost one or had to replace them because of snorkeling.
The first time we went, I was nervous and my DH was doubly nervous because he doesn't like deep water. But with the vest and the salt water, you don't go under water unless you want to. I think you and your DS10 will really enjoy it.