Ah, the fungus among us ...
Relax, Lestat. Make the appointment with the doctor, but go back to the drug store and buy something (or several somethings) with different active ingredients (actually read the ingredients). And you're not limited to gels, you could try the sprays, too. In fact, if the rash is "wet" you might do better with spray on, even if the stuff says it's for athlete's foot.
An antifungal, if it's the right one, will give you results literally within 24 hours, so try something different today and if the rash hasn't improved, try a different product tomorrow. It won't kill you, and your penis will not fall off.
And stay out of the water! Ideally, loose clothing, keep the sweating to a minimum. Heat and wet conditions are only going to make it worse.
Of course, it could always be necrotizing fasciitis ...