Sipadan is a small island, covered with rain forest. It is the peak of a steep underwater mountain which lies quite isolated, seperated from the continental shelf by deep water (probably over 1000m deep). Most of the dives are on steep walls with ledges at various depths with a larger shelf at around 30m. There is a famous cave (turtle cavern) which has stalagmites and stalactites growing inside and which is the tomb of turtles which haven’t found their way back out.
Sipadan jetty / drop-off: Steep wall with overhangs.
A nice diving website is
Scuba Dive Sipadan
Mabul island is a small oval shaped island surrounded by sandy beaches and perched on the northwest corner of a larger 2 square kilometre reef.
The reef is on the edge of the continental shelf and the seabed surrounding the reef slopes out to 25 to 30 m deep. There are several dive resorts operating on Mabul island.
Kapalai is similar to Mabul, but there is no actual island, only a large sandbank. The most well known dive site here is Manadrin valley, but several of the other dives sites are well worth a visit.