Rats are like a waterised oxymoron. They can tread water for up to three days, and yet can go longer without drinking than camels. Try just an extremely deep tray with a tiny bit of water in the bottom, see how they like walking on moisture. Gradually increase depth, relying upon positive signs. When you've got them swimming confidently, practise holding and scruffing them up, until you can wash them fully and they're happy little rodents all the while.
In the meantime, if desperate, you could try rubbing a flea powder into their fur to mask the odours. But really, if they're walking on clean litter all the time, there should be no real smell of them, except the normal fur one.