A leaking waterbed can be a real mess and a serious liability if the water makes it over the top of the liner and begins pouring onto the floor. You can buy a waterbed mattress patch kit to repair minor punctures, but the repair kits sold for fixing bicycle inner tubes work equally well. The important thing is to notice the leak--and you almost always will--before the problem gets too serious.
Locate the leak and draw a circle around it with the felt-tip pen. If the leak is on top of the mattress, you will probably be able to patch it without draining the waterbed mattress. If the leak is on the edge or underneath the mattress, you will save time and be able to fix the problem right the first time if you drain the mattress before proceeding.
Use the metal scuff pad that comes with your repair kit to roughen a small area on the waterbed mattress surface surrounding the leak. This helps the rubber cement bond properly.
Make sure the surface area to be patched is completely dry.
Squeeze rubber cement onto the waterbed mattress surface in an area about the size of a postage stamp (if your mattress puncture is bigger than this, buy a new mattress).
Peel the protective backing off a rubber patch and press the patch firmly to the rubber cement, holding tightly for 10 minutes to get a good seal.
Allow the patch to dry overnight.
Place a small piece of duct tape over the patch to prevent the corners from peeling up.