Playing golf is mentally and physically taxing on the body, therefore proper hydration and nutrition is key to performing at a high level. I recommend my players to drink something on every hole, just a sip or two. Water is adequate, but I like a supplement for hydration. I think a protein and a carb on the course goes a long way, but I think every player needs to experiment what works for them. I even go so far to have some players get bloodwork done.
Stay away from sugars if possible, they can spike energy and then the player crashes.