I guess I'll be the odd "man" out and say that my horse sees A LOT of water in the summer.
I generally bath him once a month or so, just because I feel like it [using gentle shampoo, however] and to really get all that flyspray/SWAT/sunscreen/etc residue off him. Last summer he got, maybe, 2 baths all summer. This spring/summer he's already had two [one in March, one in May].
I like to rinse this current horse of mine off every couple of days if it's been pretty hot. If it's 85*+ and he's clearly too hot [sweaty/mopey/whatever], I'll rinse him off no matter the last time I rinsed him.
My previous mare -elderly and immune-compromised- got rinsed nearly every day during the summer. Sometimes even twice a day if it was really hot. She never sweat[ed?] well and she would really overheat even if she stayed in her shed all day. She'd get confused and "weird" if she was too hot and regular rinsing would always bring her right back.
She got baths nearly every week, with "hypo-allergenic and soothing" shampoo. She would invariably rub her butt raw, literally raw, on something if I tried to skip her weekly bath. I never figured out exactly why, but a weekly bath solved whatever the issue was.
For me, it really depends on the horse.
My mare was always significantly
more comfortable after a bath, so I stuck with her preferred weekly-bath schedule. My current gelding doesn't seem to care either way, so I just stick with bathing whenever I want to.