We just picked up a couple Heritage Redfish 14 foot kayaks. I haven't even had them in the water yet.
Got them used from a friend of a friend a little over a week ago and just finished building a trailer for them yesterday. Once I get a couple new tires we will head out and grab some caches with them.
I only know of a few water only caches (I haven't looked up more yet) here in Central Florida, but I know there are some more on the coasts that we will eventually try to get.
Have you considered a sit on top kayak? I know of a few people in South Florida that take their sit on tps offshore for fishing.
A 10 foot Heritage Redfish (or something similar) would work for the mangroves and canals, but not be real good for offshore. You want something bigger (12'-14') to head offshore with.
As far as loading it on your car, there are systems where you put one end of the kayak on your car, lock it on, and then swing up the other end. I have never used that system, but I have seen it mentioned before, probably on paddling.net or Floridasportsman.com.
Good luck