The Bureau of Survey and Mapping is supposedly responsible for this..."Locates the mean high water line as recognized in the State Constitution as the boundary between state sovereignty land and uplands subject to private ownership for the Coastal Mapping Act (F.S. 177 Part II)." I can Google too!! Also check out this. I believe one has to obtain a surveyor to establish this line on your property just like a regular survey but a little more involved.
Thanks, you are a much better googler than me. ;-)