You really don't usually need to bathe a snake, & it sounds like your interference might have made him gulp or aspirate water, & snakes CANNOT cough. Snakes can hold
their breath under water (or above) for quite a long time (& way longer than we can). I think you need to let him relax. When was he fed last? Before a snake regurgitates
they often open their mouth the way you described, & again, if that's what he was doing & you interferred, you could have caused him to aspirate (into his lung) & that's
NOT a good thing, but not much you can do about it now. I hope he's OK, but just let him rest...& instead of bathing him (which is more stressful for a snake) next time do a "sponge bath" if he is really dirty.