Has your 6 year old been snorkeling before? I thought my 5 year old would love it, but we took him to Discovery Cove, and he was scared of the fish. I'm glad we tried it somewhere with easy access.
Honestly, even though he's a good swimmer, I don't think he would be able to handle extended time without standing up. Even my 8 year old struggled in the deeper areas. That was in a manmade pool where the biggest challenge was avoiding other people. I'm not sure any of my kids could handle open water with how I've seen cruise snorkel trips are described. Maybe my daughter, but she's super active in general, and wants to be a marine biologist, so there's a lot of buy in for her.