Sibuan is one of the smallest islands in the whole bay but it is very popular for swimmers and snorkelers. Sibuan Island, also called "Battleship Island" for its shiluette that much assembles a WW2 battleship, is located about 30 minutes by speed boat from Semporna. Only Sibuan Island got a gentle falling, white sandy beach that is perfect for swimmers and non swimmers alike, someone can play in the shallows without being worried about stepping on any sharp coral.
Sibuan island is inhabited by native fishers that live eating fishes and drinking boiled sea water. Nobody else could live there without fruits, drink, bar, hotels, market...(etc..) and with a burning sun on the head. Only 4 fisher families survive and to reproduce on Sibuan (there are 4 family with totaly more than 20 children!!!). Moreover the families live 2 on the island, 1 on a sort of boat house and 1 on a palafitte. In Sibuan you could live an ancestral experience with unique feelings. Moreover Sibuan island is also a quite interesting snorkelling spot. In particular you can find there the rare mandarin fishes.
I recommend to go to Sibuan island from Semporna and plain everything with Scuba Junkie a scuba dive center runs by a group of young britain divers. I recommend Scuba Junkie because other diving centers used to not go to Sibuan.
take a walk into the vilage and see the life of the local villagers on mabul island.. it's nice to be amongst them, leading a simple and peaceful life.. :)