Summers in Japan are very humid. I cycle at all I leave at sunrise (5:30) and try and be home by midday. Having got unstuck in the past I drink each time I stop - so at lights, top of a hill etc. Vending machines are everywhere so I keep two on my bike at all times. Nice and cold for a few minutes too. Typically if I cycle 100km I get through 8x600ml bottles. I drink one sports drink for 2 bottles of water. As it approaches midday I also get water just to pour down my neck. Welcome to Japan!
Note the large thumbscrew under the holder. I have the set to accept 600ml drink bottles from vending machines. Be sure that the bottle is held as tight as possible but not so tight you cannot remove the bottle. One unseen pothole and the bottle will fly out if not tight enough.