You have to bring your own towels, and you are responsible for keeping up with your own personal belongings. We bring a bag to hold sunscreen, camera, towels, and it is left setting alone at times. Nobody ever has bothered anything.
You can get a towel from the hotel towel hut as we pass by on the way to the boat. We do have a few towels on the boat, but they are to clear up spillages. Bags can be stored in the cabin or by your side. I prefer people to put them in the cabin since they create trip hazards and take up space. They can occasionally also get wet if near the front of the boat.