When we where for the last time on the island of Mljet (1999), Pomestak island was more or less with very few people there. There were a boat from the hotel Odisej to the Island, but we swam from the rocky beach under the abandoned camping (only 100 or less meter), to the most southern part of the island and then direction west. The island itself has only a few useful spots where is easy to go out from the water.
We preferred to bath and stay naked on some isolated spots at the small lake.
Hope you will have fun there. The island of Mljet is for us the most beautiful part of Adriatic sea.