Filling the water bed
Make sure you are filling the bed correctly. Remove the aerator from the spout you are using to fill the water bed, if it has an aerator. Aerators are small mesh caps on the end of a spout or faucet that make air bubbles. You don't want these bubbles in your water bed.
Fill the water bed with warm water one quarter at a time. After each quarter remove the hose from the intake valve and roll a broom handle over the mattress from top to bottom. This will help remove any air bubbles. Repeat this process after each quarter and then again three days later, just to be sure.
Once you've got the hang of the filling process you're ready to determine how full you want the water bed.
How full should the water bed be?
In the end, the fullness of a water bed is really a matter of personal preference.
Once the mattress is full lie on it on your back with no pillow. If two people will be sharing the bed, you should both lie on it at the same time. Ideally your body will be parallel to the floor when you lie on the bed. If you feel like your head is lower than your midsection, or if you are rolling off the bed, there's too much water in the mattress and you will need to drain some out. If you feel like you are lying in a hammock and your midsection is lower than your head and feet, there's not enough water in the mattress and you will need to add some.
Once you have achieved the parallel position you can make small adjustments to your preference. And once your preference is achieved you can look forward to a restful night of sleep.