lilly, I know you've probably post it before somewhere, but what age are we talking?
I only ask because if you're talking non-swimming age infant/toddler, then be sure to get a life-vest in addition to the float. Sometimes the water can get pretty 'splashy' depending on where you're snorkeling.
My DD is 3 (well, 4 April 8th) and she already has her own snorkeling gear and really enjoyed being out in the water on our last cruise. So if we're talking swimming age, take them alone, they'll really enjoy the 'fishies'.