Bonking during run

After 18 minutes, it's not "bonking" in the generally accepted, glycogen-depletion definition. Your fueling should be fine, should not be an issue in a run that short. Nor do I think it's a humidity problem, as I routinely run 5-20 miles in conditions equal to or worse than yours (no beach here!).

Eliminate the possibilities, and whatever's left, no matter how improbable, is the cause. I'm voting mental problem.

I'm in Sarasota and know of what you speak with the hot & humid conditions - it can be brutal working out in the late afternoon like you've been doing.

That being said, I think your problems may be more related to conditioning and heat acclimation as opposed to hydration or nutrition.  You aren't really running far enough for that yet. 

I always train with my heart rate monitor and find that my HR tends to skyrocket on very hot days.  Sometimes I have to slow down to a walk to keep my HR in my target zone. 

When it's really hot I try to run routes with a lot of shade.  I usually don't carry water with me unless it's really hot and I'm going to be running for 45 minutes or more.  Also, are you wearing a hat?  I've found that makes a huge difference.

In the heat you need to run slower.  No such thing as 'bonking' after 18 minutes.  You may be getting tired and hot, but you are not bonking.  I don't worry about nutrition unless I am going longer than 90 minutes.  As others have suggested, get a HR monitor.  That will give you an idea of what is going on.  My guess is that you simply lack running endurance.   This will come over time.  Yes, cooler weather will help you run longer and faster, but the heat is not the root of the problem.