The concept of "Being more prone to infections during seasons due to the dirty water " does make some sense...at least in a human concept, so why not dogs?
I've had several ob/gyn's tell me to make certain that I wash off after swimming in chlorinated water and after long hot baths, due to the same reason. It's a warm moist area that is prone to have "bacterial growths", so it is best to be safe than sorry. Why not???
I've been told that hot tubs can be some of the worst "offenders" to create problems. I've also talked with women who have had some really frightful infections after swimming in pools and lakes :(
toodles