There are three reasons why a waterbed mattress would leak.
The first thing to do is not panic. (Generally it looks far worse than it is because the water cannot go under the mattress due to the weight of the mattress, so the water squeezes up in the corners and then it will travel around the edges unless you catch it in time.)
Check the safety liner is above the water level so water cannot come out of the waterbed.
Unplug the waterbed heater.
Obtain a waterbed repair kit.
First pull back the corner of the water bed mattress and wedge a towel in each of the four corners. (six corners if you have dual waterbed mattresses) Depending on the severity of the leak this should soak up nearly all the water. If it has not, repeat the process with dry towels.
Having soaked up most of the water you can now go around and with a dry towel pull the corners of the water-bed mattress back. The corners are always the most vulnerable part of a water mattress and is the best place to start the search. Dry as far under the mattress as you can reach. An easier method of pulling back the mattress is to put your knee on the mattress about eighteen inches (45cm) from the corner. This will offer the corner up to you releasing one hand. As you dry the corners, inspect them closely for enough time to see if there is an obvious leak. So you know what you are looking for, the leak is normally little more than a tear drop. Assuming you have done this in all four corners without spotting the leak, tear a cardboard box into strips and put them between the mattress and the waterbed safety liner. Leave it for about fifteen minutes while you have a coffee and recover from the shock of having woken up in a puddle. This should be enough time for the leak to make an obvious wet patch on the cardboard. This will narrow down your search and you will then find the leak even if it means also examining the sides.
Having found the leak, pull the area right back and wedge an object in the gap to hold the leak area above the water level. A kiddies toy box is a good object to use, large enough and no sharp edges. Try to get the actual leak to sit completely flat without it moving about.
If there is a lot of water coming out, it is a good idea to open the valve and lift it clear of the water, replace the cap and work the air pocket to the leak area.
If the mattress is aging it will have started to become brittle, most likely the reason for the leak. Get a hair dryer and warm the area which will make it become supple and dry any moisture in the split.
Before putting the glue on the mattress, mark the leak with a biro because it will become invisible once you put the glue on.
Assuming the split is about two millimetres which is normal, smear the glue over the leak nice and generously about the diameter of a ten pence piece.
While the glue is becoming tacky, cut a circle from the patch about the size of a five pence piece. Do not be tempted to use a bigger patch than you need. For a longer split, cut a long oval just bigger than the split.
Put the patch over the leak. It is very important that while the glue is drying you keep touching the edges down as they try to curl up until the patch grabs and the glue is dry.
Allow about fifteen minutes for the glue to dry completely, then smear a little more glue around the edges of the patch to seal it down.
After it is completely dry you can dry up underneath as much as you can and go to bed.
Now go back to the beginning and ask why did it leak?