★★★★★
Pick an area where it's salt water, not brackish, on the flats in the Intercoastal waterway or a bay that doesn't get much run-off. Protect your feet (from oysters / shells), explore the coast and/or find a deeper spot to swim or soak in (away from boat traffic). Probably the more stuff that gets stirred up the better. It may be ewwy gooey, but it works.
I've had the sea fix it in 3 hours, consistently. The other alternatives are the over-the-counter remedies that take 6 weeks (and often fail), or the doctor prescribed systemics (2 weeks), had that fail too.
Stay OUT of low salt brackish areas, especially in the rainy season. Run-off gets in there, and it's not the sea you're swimming with, anymore.
Take care, and good luck.
Ringworm
★★★★★