There are life vests in different sizes for snorkeling. I think there is enough time for parents to take turns snorkeling at each stop, but it would not be a whole lot of time at at least one of the stops. I didn't snorkel because DH lives for snorkeling right now, and I knew he'd enjoy it the most (I got plenty of other indulgences from him, like spa treatments).
Your 3YO could swim a little in the water close to the boat with you, but I think most 3YOs (and 5YOs, unless they are pretty experienced in the ocean) would spend most of the time in the boat. The older kids would probably love the rope swing, but the younger two would probably be too scared to go on it (I don't know your kids, of course, but my 5YO DS is fairly adventurous and was too scared of it). The youngest two might be a little bored, and I was afraid mine would be miserable when I first got there, but they did have fun. The crew is good with the kids, but there's not really anything to do on the boat itself except move around a bit, eat lunch, drink juice, watch the snorkelers and people swinging, etc.
The bottom line is that it would probably be really fun for the older kids and you, not as much fun for the little ones, but I think it's very family friendly, and the younger kids would probably do fine. It would probably rank lower on their favorites list than say, a dolphin encounter. The toddler who was on our boat was extremely mellow and slept half the time on a bench with a towel draped over him. A more active toddler would be harder to deal with. DD3 is pretty mellow in group situations so was fine.
BTW, there is definitely both shade and sun, so that is nice.