My daughter has an unsafe swallow so aspiration (fluid going into the lungs) is something we were told about, especially when we were learning to feed her with an Ng tube.
Firstly, we were told that aspirating clean water is usually not something to worry about. It's not as if the inside of the lungs is dry, and only a tiny amount - if any - would get in, in the sort of situation you describe.
We were told that aspirating her feed would be more of a problem because this would provide a medium for bacteria to grow in. However even if this happened, we'd be looking for symptoms of a chest infection, which would be treated with antibiotics. NOT sudden death in the night after it happened.
In her ten years of life she has had a chest infection we believe to be caused by aspirating feed once - and it was a manageable chest infection, not pneumonia.
She splutters in the bath or pool like other children, she is at much higher risk of aspirating fluid than other children and we have only had a problem once.
You really don't need to worry about a bit of a dunk in the bath!
(The article posted above is really good.)