Tri Trang Beach is rather stony and the seabed is rocky right up to the shoreline making it totally unsuitable to bathing.
There are two attractions for this beach. Firstly, when the tide is in there is some good snorkeling. Secondly, it is the closest beach to the much more charming Freedom Beach. You can rent a longtail boat to make the trip from here much cheaper than from Patong. You may need to wait a little for a boat to arrive as they are not always waiting here but you should get one eventually.