You can get a towel first thing when you get off the ship (I've seen people take more than one, I guess they have some need for them). At the end of the day there is a bin to place them in at SB so you don't have to carry your used towels all the way back to the ship.
Serenity Bay is really all about serenity. No game areas, no stores, no paddle boat or snorkeling rentals, no music playing, no crab races, and of course no kids. They do have a bar with bar servers walking around the beach, and once in a while Captain Sparrow shows up strolling along the waterline, but other than those two the beach is quiet and filled with adults sunning, reading, sleeping, getting ready to go back out wading again. Since relatively few people are there (relative to the thousands in the family area), everything is really close to everything else. I mean really close. I think you could go from beach to bar to bathroom to bike rental to food to shower and back to the beach in fewer than 100 steps.
One interesting thing to do is stand on the SB beach and look far to the left along the shoreline. You will likely see some people down there swimming and relaxing. That is the crew beach!