Are credits cards water proof?
Up to some point they are. Under the metal pads there is usually fiber glass, then the chip and everything is covered by a glue. Then embossed on the plastic card. I would say that even if the water touches the silicon there is no problem at all. The only risk I can see is the metal contacts becoming rusty, but for a short contact with water that should not be a problem.
Another thing is the possibility of breaking it through mechanical shock in the washing machine. Cards are designed to stand some flexion and other mechanical stresses as many people just carry them in their jeans back pocket and sit down and nothing happens. I would say if after the "session" the card still works, no mechanical damage has happened and won't happen.