Jen, the Fury Catamaran snorkel on Cozumel is great. Did it last year before the hurricane, and there are 2 stops to snorkel, then they take you to their private beach. Didn't really see too many fish, but still had a great time. Crew is entertaining, it's a well organized tour. Loved their private beach. You could go horseback riding, play volleyball, swim(they provided floating mats and kayaks at no charge). I think your family will enjoy it.
We also went to Chankanaab park to swim with the dolphins in the morning before the catamaran. I'm not sure what it's like now, but it was beautiful and a great place to relax. I have been to a few different places in the Caribbean to swim with the dolphins, and this is a first class operation, and was one of the best dolphin swims, you got to do a lot with them, and you can buy DVD's of your swim and pictures. Your sons might like this. We booked this on our own and saved a lot of money. We also got to do the swim, where you are pushed and pulled by the dolphins. I don't know if the ship offers the Royal Swim.You should know that even though we managed to do both in the same day, it was very hectic trying to make it back to the ship to be picked up by the catamaran. If I had to chose only one, I think I would pick the Fury Catamaran. However, we just got back from Costa Maya 3 weeks ago, and I noticed they had a Fury Catamaran snorkeling tour there, so maybe you could do that in Costa Maya if you don't end up doing that in Cozumel. Just a thought.
As for Costa Maya, there really isn't much to do there except the Chacchoben Mayan ruins and the beach. The shops at the port are kind of nice, and there is a pool at the port, but we didn't use the pool and had very little time to shop. We took a private tour given by two local men who grew up in the area (David and Ivan). It was very well run and they used clean, air-conditioned vans. My DH and I enjoyed the tour and loved going to the jungle, however our 4 daughters (ages 13 - 24) didn't really appreciate it and would rather have spent the whole day at the beach. So even though we saw many kids of all ages there, only you know what your children will enjoy. I'm not sorry we took them, though, because beaches are all over, and I wanted to expose the girls to the ruins and another culture. The Mayan culture is studied in school quite often. If you are going to make it a beach day, I think I would pick one of the beach breaks. We only had 2 hours to go to the beach, so we went to Mahahaul. It was okay for a short time, but to spend the entire day, I think it would be nicer at a beach club. I didn't like the atmosphere as we walked up and down the beach, it was okay, but not what I like to bring my family to. All the same, I would go to Costa Maya again, I would just pick a different beach. Just to let you know, some people really love Mahahual and rave over the town, it just felt kind of tacky to me.
Hope you enjoy. Sorry for the long post!
Lynne