Brad:
I don't know of any rashes cintracted in the tropics which don't go away for a year. The original rash sounds like what the locals call "sea lice." (Actually, they're small hives from the sting of jellyfish larvae which gatger under bathing suits.)
I have never seen these persist, however. Obviously, the dermatologists you saw did not recognize the rash, nor did their biopsies shed meaningful light.
Is your wife possibly perpetuating the rash by scratching all the time. Are there still hives and welts, or just areas which have been rubbed or scratched?
If she is not doing so, I would recommend at least these symptomatic treatments: One Aveeno oatmeal bath for 10 minutes at bedtime; Benadryl 25-50 mgs at bedtime, application of Sarna lotion (OTC) liberally and often to suppress itch.
Good luck.
Dr. R