We were last in Bintulu and Kuching about four or five years ago.
Bintulu is a large commercial basin with little swinging room or place to anchor. We dropped the anchor following instructions from the marine police and they took a line from us and tied it to the commercial concrete pier. It was okay for the two days we stopped and went into town, but Peter resecured the boat using chain to the concrete so it wouldn't chafe through our lines. I wouldn't leave the boat there. Storms in that area can be quite violent, and there was nothing there to secure the boat. http://www.cruiser.co.za/hostmelon22.asp
Kuching, to get up into the town one must get your mast under a bridge. I have forgotten the height, but it was not high enough for our mast to get under. It might be 55' high, but do not take my word for it.
Otherwise, the boats continue up the river and anchor near the cement plant. The current is swift, sailboats only go up river with a rising tide, tidal range is between 2.5 and 5 meters depending upon moon phase and season. There is a place where the fishing boats refuel and get water, but very few private yachts go up the river. http://www.cruiser.co.za/hostmelon24.asp
Sibu was even more difficult and insecure.
Cruising hasn't reached these places yet.
You will enjoy the area, and will find much to wonder at, but you need to take your time and be careful.