I assume this is the same Blue Lagoon as the Blue Lagoon Dolphin Encounters.
I did the Dolphin encounters a number of years (6 maybe) of years back. When you do the dolphin encouters, you get to use the beach as well. The beach was nice. The whole island is nice. There is a (expensive) snack bar, and you can walk around and look at the various sea life they have there. The beach is in a lagoon, so it is calm, and there are floats and such that you can use. One thing that I REALLY liked about the Blue Lagoon island is that they had boats that ran back to the cruise port about every hour on the hour. Some island "hold you captive" you get there, and then it is 4 hours (or more) until the next boat, and you are stuck hanging out, wasiting port/ship time, and being pushing into buying their overpriced merchandise while you wait for a boat back to port. Blue Lagoon had regular boats back to port - at least 6 years ago they did.
-dave