I see that you will be doing the land and sea. We did the catamaran sail and snorkel. We had a great time. It was the first time for my sister, her husband and kids (12,7). Even if you just float and put your face in the water, it is beautiful. They have all the equipment. They give instructions before you go out. You will be back around 1 or so.
The year before, we went to the Blue Lagoon. There is some snorkeling there. If you don't have equipment you can rent it. I don't remember how much because we had our own. You must have life vests-I believe these are $5. This was our 1st time. The nice thing here is that if you don't want to snorkel, there are other things to do. There is a very shallow inlet to play in. You will not be back until around 3:00.
Have fun!