Just captured this picture yesterday and oh boy the rate is so cheap. "Tambang" is a small boat service to cross the Sarawak River. It is a convenient way of commuting to and fro for both sides of the river. Each side of the river has its own attraction. One side is water front and the other sides are mostly residential area, new Dewan Undangan Negeri (DUN) building, Palace, etc.
In case that you have the opportunity to travel to Kuching, make sure that you try the "Tambang" boat. Most of hotels are located near the water front and the Tambang is a convenient way to walk around the city and visit the new DUN building.