Daz,
You're gonna think I'm totally nuts but here goes!
There is a company called Amina
www.amina.co.uk who produce a product that can turn an accrylic bath into a speaker. It looks like the product comprises a fully waterproof control panel (toch sensitive) that you can install in place of a tile, and a unit that fixes to the bath. It's priced from £300, and the product is called rhythmBath.
All you then need to do is to connect this up to an audio source and off you go.
My source for this info is this months edition of Homebuilding and rennovating magazine.
Hope this bizarre information is of some use to someone!!!!
Tony