We did this last year and it was fantastic. Get there early morning before opening time as it takes a little time getting sorted out. We had a choice when booking whether we wanted a slot in the morning, mid morning, early p.m. or late p.m. We had to choose two slots as we couldn't all swim together due to having a little one with us, so we choose mid morning and early p.m. Its not till you check in that you are given an actual swim time within your timeframe slot, as we checked in early we had the pick of the times.
I would highly recommend getting a wet suit as the water is cold. For children you definitely need the swim vest as you have to go into quite deep water. Don't put on suntan cream before you go as you are given special cream which is 'dolphin friendly'. You are also given snorkelling equipment which you get to keep. Feeding the fish is definitely an experience not to be missed.
The lunch there was one of the best I can remember with lots of good choices and plentiful.
Its a great day and you'll have a fantastic time.
Annmarie