I am no expert here, but I have installed three of these units. I use Takagi brand TK-3 model. They come for either propane or natural gas. You need to first determine your peak GPM usage, and also your temperature rise. In Florida that would not be as high as up north.
One thing to know is that you need 3/4" inlet for Natural Gas.....a tank unit usually only needs a 1/2" gas line.
For an electric tankless water heater, you need your main service to be 200 amp minimum. The ones I have seen take THREE double pole 40 amp breakers...that is a lot of electricity.
One minor downside to these heaters is if you lose power, you have no reserve hot water. Also, they are quite complex on the inside, so if a lightning strike blows a circuit board, you could be without hot water until you can get another circuit board installed, as they are much more complicated then a standard water tank.
BTW, to the poster who asked about pilotless gas water heaters....they do make them.
Here is a picture of one I recently installed;
Note that this was up north, thus it was in a basement. One more comment. I notice that in Florida many times you cannot see the end of your T&P valve extension. Code states you must be able to see it, so you can tell if it is leaking. I could not tell for sure, but the picture of the one in an earlier post does not look like it terminates in the same room, 6" or less from the floor, as code specifies.
Frank