Summers in Japan are very humid. If I cycle outdoors 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 rather than the typical water bottles that you see on most bikes I opt for a Topeak Adjustable Waterbottle Cage with two nice cold drinks from vending machines. Vending machines are everywhere in Japan, and even in mountainous area I'd expect to be not more than 15 minutes from a machine.
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 them 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 so it's part of my leaving the house routine to check they are ok.