It can vary a lot horse to horse. Also, some horses sweat more than others, which is a factor as well.
You might see if your horse who's less interested in drinking will drink gator ade? And if so, pour some of that into his/her water bucket? We do that at shows to be sure everyone drinks plenty and they beg for "gator water" like it is cookies!
Another thing is I used to have a horse who would drink "too much" for a period of time. He would go through more than one 5 gallon bucket of water in the 12 or so hours he was stalled each day. We started putting electrolyte powder in his water and he eventually drank a "normal" amount (about the same as the other 11 in the barn). With him, I wondered at the time if it was fixing to be some kind of a vice. He didn't come from the best situation when I bought him (full of worms, way under weight, no energy, dull...). Speculate that he didn't always have nice, clean water and wondered if he was going to just drink "too much" water for something to do in his stall, etc. Happily, though, he moved through it and stopped. Sure makes stalls a nasty mess to keep clean.