Well, I'm not sure what your weather will be like, but when we went in June it was very nice. We enjoyed the Blue Lagoon excursion. We found it to be very peaceful. We just relaxed for a few hours on the beach, did a little snorkeling (our first time, its nothing fancy by any means) ate lunch (the food was good, the punch was horrible!!!) Yes you do have to walk a bit to and from the boat to the ship. Personally, I didn't think it was bad. I felt like I was getting a little exercise after all the food. I didn't like walking past all the taxis and the hairbraiding ladies. I'm very glad we didn't have to deal with them to go into town.
All in all, its one of the least expensive excursions so if you want to do something for a few hours, this is a nice way to spend it. We had our choice of return times 2:00pm or 4:00pm. We did go early so that we could all shower (not used to the salt water). We have not done alot of traveling so I guess I'm not a good person to compare things with. We will be returning to the Blue Lagoon in June. However, if I thought that it was going to be to cold to swim, I would cancel. Hope you enjoy your trip!!