Man, derawan/Sangalaki/Kakaban was one of my favorite places visited yet. I had to take a visa run from Sipadan/Mabul area wher eI was working at the time, and decided to visit Kalimantan. Took quite a journey to get there (Ferry, speedboat, taxi flled with durian, mini-bus, speedboat) but when we got there... bliss. In one day we snorkeled with about 20 green turtles on derawan, went diving barracuda point kakaban where we saw white tip sharks, black tip sharks, and spotted eagle rays, then surface interval in the jellyfish lake (mind blowing). on the way to sangalaki we saw a devil ray on the surface, a pod of dolphins, and of course 10-15 mantas at sangalaki. (Not to mention the rhobust ghost pipefish). The boats are tiny and uncomfortable and you basically have to convince the DM to take you diving, but man is it worth it when you see schools of manta rays and not another diver or dive boat in sight. just me and my girlfriend.
The reefs around sangalaki are in bad shape because of crown of thorns, but the mantas are still around. Hopefully it starts to recover soon.
-Nick