I know people here have rented from caremedical before. Are the manual wheelchair seats vinyl or nylon (plastic or cloth-like). A vinyl wheelchair in florida in summer will get hot!
I don't know for sure, but most of the rental wheelchairs I have seen had vinyl seats.
What a lot of people do is bring a towel along with them to put over the seat.
Go to Sears... they make a wonderful seat cover for lawn tractors, it keeps the seat cool in summer, and warm in winter. Cheap too. It slips on easily and rolls into a small ball when not needed for storage. HAVE A GREAT TIME.