Or you could try a smaller water bowl.
Evaporating water - ventilation = humidity
Fix either of those two issues on the left side of the equation as needed to adjust the right.
My hogs experience higher humidity during our summer months, and don't suffer for it, but it's a good thing to keep an eye on. Another thing you could try is using care fresh bedding. It might lower the humidity just a tad because it's so absorbent, and hoggies love to burrow in it.