I've always sweated more heavily than many of my peers during even fairly moderate athletic activity, and on a humid, even moderate-temperature day I'll finish a long run looking like I fell into a pool. In non-athletic contexts, I also sweat more than some seem to, but not to excess, i.e. it's not socially awkward and I doubt people I encounter non-athletically think of me as a noticeably heavy sweater.

I'm fairly lean, and it wasn't much different in the past when I was extremely lean, so I don't think it has anything to do with body fat. It also doesn't seem to have much to do with fitness. I'm certain there's a strong hereditary component, my dad has always been a heavy sweater, for example I remember him carrying a sweat cloth during hikes together when I was younger, and up through the present day.

It's my impression heavy sweating is inefficient. My impression is that an optimal sweat rate results in rapid evaporation of sweat to cool the body, and sweating in excess of that cools only very inefficiently and at the cost of water and electrolyte loss.

During workouts and races, I inevitably become somewhat dehydrated, because I can't absorb enough water to match my perspiration rate. If I do drink closer to maintenance levels, then I just have to pee. I've experimented with salt tabs, on top of the electrolytes in various caloric and non-caloric sports drinks. I'm not sure how to think about appropriate salt dosing, it does seem to help a bit in retaining fluids, but only marginally.

I'm sure there are others on here with similar issues, so I'm curious if others have strategies they've found helpful.