I can't really help you with the waterproof stuff - I just go into the bath for watersports. But for the sheets, this is what I do:
I bought lots of cheap flat sheets at Asda in a cream colour. About a dozen. And several extra pillow cases. When a client comes in, I put a fresh sheet down on top of the duvet. So, I change the sheet for each client and can change the duvet cover when needed. (I also have a separate duvet for my personal use when I sleep over). I change the pillow cases frequently as even though I cover them with the sheet they will often pull them out and position them to suit themselves, so I will change those if I think they're dirty or have even a whiff of sweat on them.
Really cheap sheets work fine for this and if they wear out, who cares? They've very cheap to replace.
Very high thread counts may pill. They also will take longer to dry.