At Sabah, there is a jetty known as Jesselton Point where it is the gateway to many island hopping adventures. I think you can get many packages from the tour guides here and they are more than happy to work out a plan. We had a group about 30 pax and ended up going island hopping to Pulau Sapi and Mamutik. Sapi Island is definitely bigger with more amenities but its also because of this that the waters surrounding the island is also not in as good condition. Mamutik on the other hand has better water, lots more fish as well as corals. Do check out some photos. And sorry I don't have an underwater camera so don't expect coral pictures from me.
Archway to the jetty
Interesting street art on the way to the jetty