My collie bitch is always desperate to reach water in order to dip her butt, it seems to give her immense relief yet we can see nothing wrong. She doesnt scoot and Having her anal glands emptied makes no difference. After her dip she is a different dog but after a little running she does it again but never swims. I bathe her anus frequently hot/cold compress, Epsom salts but nothing changes. Vet can see nothing wrong but glands often impacted even though stools are hard enough for natural emptying. She is a very smelly dog, worse than normal doggy smell. I can find no reference to this anywhere.
Did you ask your vet to also make a vaginal swab test? Maybe you could ask for this specifically.
Female dogs do have vaginal infections, there's not always any obvious sign but a larger red vulva and/or colored discharges, and it's very smelly! The dog may have some kind of weird "metal" smell because of the bacterial infection, similar to the smell of a (very) badly infected open wound.
A veterinarian can do a swab test; this test is not stressful for the animal and he can give you a diagnosis in a few minutes usually. He might also choose to do other extra tests, but from experience they usually investigate the issue if the problem persists. If your dog has this kind of infection, the vet might prescribe antibiotics; it's quick, inexpensive and easy to fix.
I suppose the veterinarian already investigated the issue of anal gland infection or abscess, and it's not a situation the dog would tolerate a long time. Vaginal infections can go easily unnoticed for a few weeks. Finally, there's also anal micro-fissures that could be a cause, and worms but that's probably one of the first thing the vet checked.
There's most likely a physical reason why your dog seems to want to get relief from putting her bottom in the water!