Florida is a big state— the distance from Jacksonville to Homestead is comparable to that between San Francisco and Los Angeles, or Washington to Myrtle Beach— and so weather and water conditions will vary a good bit north to south as well as on the Gulf of Mexico side as opposed to the Atlantic Ocean side. In fact, Henry Flagler had built hotels in Palm Beach, intending it to be the "freeze-proof" terminus for his railroad. But a severe frost in 1894 wiped out the citrus crop there, and so Miami became South Florida's main resort town instead, only 50-60 miles to the south.
Fortunately, NOAA's National Oceanographic Data Center does publish water temperature tables which make it easier to compare than looking up current readings from WeatherBug or some such. To cover Florida, have a look at
As you can see, ocean surface temperatures in the Virginia Beach area (Chesapeake Bay Bridge Tunnel to be specific) range the mid- to high-70s Fahrenheit in late summer. No place is quite that warm on the Atlantic side, except down in the Keys, but the Gulf side at least seems to be reliably balmy all the way up and down.
I'm not aware of anyone who measures water temperature of small freshwater bodies, but since you won't be immersed in the water when canoeing, I would hardly think a paddle through the Everglades would be chilly.
