If you hear the water inside the mattress it's saying "Burp Me!" You want your mattress to be quiet. An air bubble floating on top of the water will make an annoying sound and subtract from the floating sensation.
Water is made up with air so it's normal on occasion to accumulate an air bubble, certainly after filling your mattress with a 50 foot garden hose......
(A)... To burp your mattress just throw it over your shoulder and pat it on it's back......No, use a broom stick!...... Open the filler valve and pull it up a few inches...... I like to wrap a towel around the filler valve just in case you spill some water......
(B)...Next go to the head of the mattress and using the broom stick...... Gently push and force the air bubble toward the filler valve...... When I add my yearly conditioner to the water, I will check my mattress for any air bubbles. That's about all the maintenance to your waterbed mattress other than cleaning the mattress with a little soap and water.