There are a couple of reasons you may need to siphon water from a waterbed mattress. You may have overfilled it during installation, need to move or dismantle the bed or repair or replace the mattress. Most waterbed mattresses come with a fill and drain kit, but if yours didn't, you can purchase one at a home improvement store or big box retailer. It is not recommended that you skip the fill kit and try to suck the water out of the hose with your mouth; garden hoses contain germs, and siphoning with your mouth poses a drowning hazard.
Turn off and unplug the waterbed heater. Run a garden hose from the bed to the nearest sink faucet.
Pull the mattress fill valve out of the mattress and remove the cover. Place the handle of a broom across the opposite end of the bed. Run it along the top of the bed, pushing any bubbles out of the fill-valve opening.
Screw one end of the fill kit to the faucet and one end to the garden hose. Set the fill-kit dial to the "Fill" setting. Screw the other end of the garden hose into the fill valve on the waterbed mattress. Do not to allow any air into the valve.
Turn the faucet on and listen for all the air to release from the hose. Leave the water running and set the fill-kit dial to the "Drain" setting. The water from the hose is diverted into the sink while creating a siphon that sucks the water from the mattress.
Turn the faucet off when the desired amount of water has been siphoned from the bed. Detach the hoses and fill kit.