Air out a new waterbed mattress for a few days before placing it in its frame. Open it out fully and air it indoors. Preferably, do it outside on a dry lawn or clothesline. Much of the smell will dissipate.
Ventilate your bedroom. Operate an air-exchange system or open windows as the weather permits.
Expose it to fresh air if it was stored in a very hot place. Excessive heat sometimes causes offgassing to increase. The exposure will bring it down to a cooler temperature.