I am from (born, raised, lived there until I was 32) a region with high, humid temperatures and have now spent 2 years in an extremely dry climate that still gets quite hot in the summer, even though my entire life I've struggled with dehydrating very easily and quickly.
When you are experiencing extreme heat, your body is working overtime trying to keep you cool. You are using up all your nutrients and energy faster in addition to losing electrolytes as you sweat. You need extra water, extra electrolytes, extra sugar (your body converts sugar into energy and boy is it burning through it's energy reserves rn), and extra nutrients like vitamins/minerals.
-- The reason tea and coffee are said to not be hydrating is because caffeine is a diuretic, which means it makes you pee. It is a myth that they are not hydrating because they really do have plenty of water content, but also you should be mindful of how much caffeine you're drinking. Consider decaf/caffeine-free options for your additional cups beyond your usual amounts so that you are not having to pee too frequently.
-- I have never heard that milks, coconut water, or juice are not hydrating, thank goodness. They very much are. In fact, they all have water AND other sugars/nutrients that you need. Drink them.
-- "Watery" fruits/veggies are extremely helpful during extreme heat. They really do have water AND other nutrients that your body needs. All the ones mentioned above are great. I particularly like melons (watermelon, cantaloupe, honeydew, etc.) for their water + sugar content. Another thing I do is freeze grapes. I portion them out into separate containers, then grab one to let it mostly un-freeze before eating. This was great to take in my lunches to school as they were the right level of unfrozen right at my lunchtime. Just don't let them completely unfreeze or else they get mushy.
-- Electrolyte drinks, like Gatorade, Pedialyte, or salt packets (LMNT, Saalt, BioSteel, etc.) are extremely important. As said, you need to replace your electrolytes. You should absolutely be drinking these AND plain water/other hydration sources. Please.
-- Popsicles are also an excellent way to get some more water/sugar while cooling off. If you don't have a popsicle stand/kit to make your own, you can make ice cube versions; pour your juice/liquid of choice into an ice tray and then put in any fruit or inclusions if desired.
wet a towel or cloth with cold water and then place over your forehead and/or across the back of your neck.
swipe a cool, wet cloth over your body and then let your skin air dry (it's the same physics as sweating but loosing less of your own water/electrolytes from it)
remember that hot air rises; while inside, spend as much time as possible as close to the ground as possible. It's often not much, but any little bit helps
be intentional about moving as little as possible-- bring entertainment, snacks, and hydration to wherever you're going to sit/lay and then stay there as long as possible. Conserve your energy.
half-fill water bottles and then lay them on their side in the freezer. Once frozen, fill the other half with water and drink. This makes sure the water stays more evenly cold bc the entire thing is in contact with the ice.