Derawan is a very relaxing small island with lots of turtles, we saw one big female turtle laid egg at midnight and on the long wooden jetty you can easily see tens of turtles swim by.
Underwater it was mostly about small critters and turtles, it was not particularly amazing.
There are three islands close to Derawan which make the diving amazing : Maratua, Kakaban and Sangalaki.
Kakaban Island is the place with a jellyfish lake, they were some good drift diving as well at the Barracuda point.
Sangalaki was famous for Manta ray congregation, but I saw only one from the boat. Underwater we saw some leopard sharks.
Maratua is 1.5 to 2.5hr of boat ride from Derawan but it was a place for pelagic, we saw endless barracuda at the channel (Big fish country) and a really big gray reef shark patrolling. we also saw some eagle rays at close range and I heard that it is possible to see thresher sharks.
That was my experience way back in April 2007.