Heat + Humidity = Wall

[OakLeaf]

In humidity, arm coolers aren't any cooler than bare arms, but they ARE cooler than skin slathered with sunblock that keeps you from sweating and cooling yourself at all. If you use sunblock, I highly recommend ditching that at least on your arms and using arm coolers (or a bolero) instead.

+1 on better hydration, and that includes electrolyte replacement. Doesn't sound like you got much salt at all. Remember that you're losing a minimum of 500 mg sodium per liter of water (half hour to 45 minutes of riding in that kind of weather, most likely) and your body can't absorb water without salt. My electrolyte replacement of choice is Nuun U Naturals, but there are a lot of good options, most of them more easily available at your LBS, and even if you're just buying at a convenient store, you can buy a bottle of Gatorade, swipe a salt packet off the hotdog display and dissolve all or part of it into the drink bottle. If you aren't thirsty in those conditions I'd bet my house it's because you're hyponatremic.

A hydration pack has pros and cons. I will definitely wear one in hot weather if I can't carry more than two water bottles, but in Florida where there's practically nowhere you can go without passing at least a convenient store or something, you may do better just stopping frequently for refills. Obviously wearing something on your back prevents you from cooling yourself on a large part of your body.

Which is my same experience with those gel bandannas. They're nice and cool on the outside if you reach up and touch them with your hand, but the heat exchange in the gel is very poor, so all they do is cover yet another part of your body and keep you from cooling yourself. The inside that touches your skin just gets hotter and hotter, and so do I. If sweat isn't evaporating off your bare skin faster than you can sweat, which is pretty much inevitable in humidity much lower than Florida's, then external evaporative cooling won't do a thing for you and will probably make you hotter. On the moto I have had excellent results with one of those thinner evaporative cooling vests, even in Florida, but we're talking about air flow at 45 to 75 mph (under an armored mesh jacket of course). I really don't think it would be any use at bici speed, to the extent that I've never even bothered to try.

If you have the budget, you might try one of those solid-to-liquid phase change cooling vests, but I have no experience with them and can't comment. I'd look for opinions before buying one, but I personally am willing to spend a lot when it comes to safety. Dizzy and disoriented on two wheels in traffic is to be avoided at all costs.

Do get your hair wet whenever you have the opportunity. Tousle your hair under running water, or in a sink if you find one clean enough, to get your hair good and soaked. Won't help for long, but it will help a lot.

Good luck and be careful!