I'm going to be in Mabul - Sipadan in May this year and it is my second visit this year. To answer your question you will be fine. Dive within your limitations and when they ask you if you want to go into Turtle Cave say no. There is great beginner diving al around the island with plenty of turtles, dolphins, barracuda, rays, manta rays, reef sharks, the odd hammerhead shark, clownfish, giant clams..... The list is endless. When I'm there I dive with Borneo divers and a chap named Clement Lee. As a beginner diver I would recommend you probably not dive in the cave or on any deep wall dives as someone stated above it is very easy to go deeper then planned on a wall dive and when the bottom around the island bottoms at (what I'm told) is 600 meters you really don't want to hit the bottom. There was also a really exhilerating drift dive we did with hundreds of reef sharks. Have a great time and enjoy Borneo it is a great if not one of the best dive destinations in the world.