The gas wouldnt' affect bub but the morphine would. In fact you wouldn't be allowed to have morphine in the water in any case.

If you truelly want a waterbirth and your birth centre doesn't "allow" it, there are a few things that you can try.

You can position your bum over the plug so no one can get near it. Or you can take your own plug from home in. Legally the nurses and Dr's are not allowed to touch your belongings. So if they try to remove your plug from the bath you can have them charged. Not saying that you have to go this far but it's food for thought.

Also if you are really wanting a waterbirth is it possible for you to find a hosp that "allows" them nearby ? OR.... You can always have a homebirth