We did this excursion two months ago and had a great time. Here's my advice:
Go ahead and book the dolphin excursion for those who want to "dolphin" and the Blue Lagoon Beach excursion for those who just want to "beach." When we went, the two groups both met in the same bar and left the ship together. Both excursions are located together on a very small island.
You'll ride together in a double-decker ferry out to the island. Expect a 15-minute walk to the boat, then a 45-minute ride each way.
When you get to the island, those who are doing the beach thing will be directed to a BEAUTIFUL beach on the left side of the island, while those who are visiting the dolphins will be directed to the right hand side. The whole island is SMALL, so don't worry a bit about meeting up after the dolphin excursion is finished.
When you're finished with the dolphins, you can join your family on the beach -- this is free once you're already on the island. The island was not at all crowded on the day we went. Do not do what some people in our party did and pay for both excursions! Hammocks were available in the middle of the island, and we had no problem getting a picnic table by ourselves for lunch.
Before we left the ship, we were all given tickets for lunch at the Garden Grille, the only place to eat on Blue Lagoon. The dolphin excursion didn't mention food, and it may have been a mistake that we received tickets -- we didn't ask. They serve hamburgers, hotdogs, chicken, mahi-mahi -- maybe other stuff. The food was okay, but the drinks were defiantely lacking. Bring cash to buy sodas, or bring along water bottles.
The ferry will make return trips at 2:00 and 4:00, so you can plan accordingly. We took the 2:00 ferry, as did most of the people on our trip. I think lots of people were planning to go to Atlantis in the evening since the ship doesn't leave until late at night.
As for making reservations, do consider two things before you book with an outside company:
1. Disney's excursion does cost more, but it includes two fairly long ferry rides. I'm not sure how you'd get to Blue Lagoon if not on Disney's chosen carrier. Also, if you choose to do your own transportation and you're late, the ship may leave you. Personally, I wouldn't worry about this if I were on foot or near taxis; I'd keep an eye on my watch and I know I could make it back to the ship in plenty of time. Counting on a non-Disney ferry would make me a tad nervous.
2. If you book with an outside company, you're going to have to pay them with a credit card up front. If the weather is bad, you're paying anyway. With Disney's excursions, if a surprise thunderstorm arrives, you don't go, but you also don't pay. Actually, the dolphin experience is under a covered awning, so you'd probably be okay in a light rain. The beach day, however, would be ruined.