Haven't done any of those excursions, but a little tip about taking your daughters snorkeling. I believe when doing a tour and even not, they will provide vests to wear while snorkeling, I always wear one, so that will help a lot. Its just nice added protection in case you become tired while out there and a lot of the time, I have no idea where I really am.
Have a great cruise on Constillation, we were thinking about that one, but have booked Radiance of the Seas, very simular itinaries, but this one left on a Sat and worked out better for us. I have not even started looking at excursions, there are so many to choose from.
Let us know what you finally decide to do.