Crystal, Tibbe says to tell you this shampoo-and-water session obsession is a human foible that dogs just must learn to endure since they visit it upon you with great regularity, whether you need it or not! He says learn to grin and bear it and let them have their jollies as you doggies always wind up finding ways to get more than even. He says just learn to act calm for the duration and use your energies to plan your revenge.
(P. S. Crystal, as Tibbe's mum, I for one can attest to the getting even part!) 