We did this excursion last week on our Dream cruise in Nassau. We went early in the morning - met at 9:00 and got to Blue Lagoon about 10:30ish? Blue Lagoon has three times to go back to the ship - 1pm, 2pm, and 4pm. If you booked the 12:30 excursion, I am 95% sure you would take the 4pm boat back to the dock and then go straight to the ship. I would say the encounter took about 30 minutes? We had about an hour on the beach afterward and took the 1pm boat back. You should be able to get some time on the beach

We really enjoyed ourselves! I'll give you a tip and let you know that you can purchase your photos online off their website after you get back so you don't have to spend time waiting for them there! Have fun!