Actually it's at Mamutik and Sapi islands, just few minutes by boat from Kota Kinabalu. Those are fairly nice small islands with quite clear water (not top-transparency but managable) and pretty large number of colorful fish that can be observed while snorkeling. We spent there one day, first half on Mamutik island, which is somewhat bigger, we had lunch there, and then we went to Sapi island in the afternoon - it's smaller but there are more (dead) corals and more fish.
There's not much to describe here, we just regret that we didn't have underwater camera to take photos of the fish. Also, it was the first place that we learned how to float and snorkel :)
After we came back - we passed by a wet market just in front of Le Meridien hotel in Kota Kinabalu - it's full of ultra fresh fish of all kinds, including big tunas.