Normally I would say every couple weeks is too much, however a marsh is much different than a lake or stream where water has some movement and is overall decent quality. I would expect a marsh to have a much higher content of bacteria and nasty things in it. So, if he needs it, then bath but make sure to use a dog specific dog shampoo- they are made to not strip oil from the coat. People shampoo, dishwashing soap and even some other animal shampoo are made to strip the oil. And I would use a dog specific conditioner afterwards. When I was showing and on the show circuit many of my dogs were bathed (dog shampoo and dog conditioner)on a weekly basis and they had no skin issues or dry skin. I agree with high protein and high fat- that often has much more to do with dry skin than bathing. Make sure to rinse extra well- shampoo left on will look like dandruff.