DH drinks a ton of water. I encourage him to wear cargo shorts. Men's cargo shorts can reasonably carry several water bottles.
We leave the resort with more water (in summer), and drink one early. Maybe while walking down Main St. I'm not one to stand in line just for water, but at every meal, at least in summer, we get (extra) water with our meal. We are not above refilling our bottles with leftover ice water from a meal, or carrying a paper cup for a bit.
I'm with you on sunblock. Last trip, I found a sunblock stick. We apply a good dose before we leave the room. The stick is largely for back up. Walking around, my legs have never gotten burnt, mostly my face/shoulders. On prior trips, I found a tiny container/tube and refilled each night as needed
My cell phone is kind of my everything-in-one now: electronic copies of conf #'s and receipts, GPS (w/car), camera, phone.
Mostly plastic ponchos will fit in a back pocket. If you ever end up with a jacket, zippered pockets are great. I have a windbreaker from LLBean (technically it is not a rain jacket) and it folds down into it's own zip pocket. Love it at WDW.
If I have to carry a bag, I like the Dooney and Bourke crossbody pouchette. It is so light I don't notice it, even in summer heat. It tolerates getting wet. On rides, I don't have to do anything with it. Just big enough! I also hate having any bag at bag check, but this has only ONE main zipper compartment. One zipper, and the guard can see the whole bag. The inside of the bag has pockets, so items like chapstick can be kept organized. Holds everything BUT water.
Last summer, we had a combo all over FL trip planned....we broke standard protocol. I normally don't bring a backpack, but for that trip- I found a small backpack cooler at Target. The brand is embark. While it is a backpack, it is small, with padded straps. It worked great at the beach/hiking. We also used it one day at WDW when realfeel temps were over 100. Since it was small, it was way better than the average backpack.