Mabul is a little island some 25mins by boat from Sipadan. Many who visit the area stay either at Mabul or at Kapalai. You may have found out by now that Sipadan no longer caters to overnight stays. All operators who had resorts on Sipadan have now relocated their operations elsewhere.
Mabul itself is a small island, currently with 4 resorts in the area, 1 converted oil rig stationed just in the shallow waters off Mabul , a bajau fishing village, a mosque, a school and an admin centre for the village. Just to the left of Sipadan Village Resort are several stalls selling all sorts of seashells. Please do not buy them as you may have been informed that it is an offence to take anything of this sort out of Malaysia and could be an offence to take them into your home country too.