I don't think the water should ever be deep enough for them to "swim" in when your giving them a bath or washing them. Just fill the sink, or a seperate tub (that's what I use) with just enough water to *almost* cover your snakes body. They shouldn't have to try and swim, although I'm sure they can. If it's lukewarm water and they aren't trying to stay afloat, many times they will relax and not spaz out during "bath time".
I think it was mentioned before, but they don't really need baths that often...a bath is usually to hydrate a snake whose enclosure isn't humid enough or is otherwise dehydrated.
To the OP, if your mom is really worried about it, like the others have said, lots of hand washing, but you can also take a warm damp towel and let your BP slither through it when you first get her out. She will be a bit cleaner that way to help ease your mom's worrying, and it won't be as stressful as a bath. ^_^