We did the blue lagoon 2 years ago. There are alot of opinions about this one. We really liked it. We played in the water, snorkeled (you must have or rent vests there to snorkel-I think its $5??) and ate lunch. The food was ok, the punch was awful (at least it was then). I'd suggest bringing drinks and maybe order cookies from room service (there wasn't anything sweet when we went).
You meet around 9:15, get checked in, then take about a 15-20 minute walk to a boat. Both the dolphin swim and blue lagoon day go over together. It is about a 45 minute boat ride to the island. They have a cash bar on board I believe. We were given 2 times to return (2 and 4). I think you must use cash here. They do take pictures here, but you must buy them before you leave ($10).
In our excursion book it says that the amenities of the Blue lagoon beach day are included in the dolphin encounter. Towels are given when you meet on the ship in the morning.