If its only one or the other, you would do MUCH better to hunt over water in the afternoon as thats when its HOT and the animals are searching for water. In times of extreme drought this tactic only gets better. When your area has had an extremely wet year and water is everywhere, dont waste your time hunting waterholes. You are better off looking for the enhanced growth of browse or crops under such conditions.

Most " crepuskular animals" (lmao, I think thats how its spelled. Anyway that means early/late movers like most big game animals.) wont require much water in the A.M. because of the moisture they obtained while grazing/feeding on dew covered food. After laying up allday " chewin cud" they need some water before reloading their stomach late in the afternoon and into the night. This is when the waterholes are good.