The one day trip we were most excited about on our stay on Derawan was going to Kakaban, an atoll about 45 minutes by speedboat that houses a large salt water lake packed with sting-less jellyfish. You can snorkel and swim with them and seeing them floating and paddling was super-awesome and magical. The lake is totally packed with them, as soon as we jumped of the jetty, we found ourselves surrounded by the jellyfish. We swam around very carefully trying to avoid bumping into them. The jellyfish seem a bit fragile and flippers should not be used in the lake.
There are four different kinds of jellyfish in the lake. We definitely spotted 3 kinds, two with brownish arms and an almost fully transparent kind. In the video below there is also a very tiny jellyfish zig-zagging around that has no arms at all. Since it was very tiny, we thought it might be one of the brownish ones and the arms will develop a bit later.
The place is one of the most favorite tourist spots in this part of Kalimantan. We were lucky to start early in the morning with the speed boat. When we arrived at the lake, there were just a handful of people on the jetty. After snorkeling for about 90 minutes, we were surprised to see about 50 people on the jetty and in the lake. It did not really matter though, since the lake is quiet huge. However, a quiet surrounding fit the slow and elegant jellyfish much better.
Here is a short video of swimming with the jellyfish.