pinellas, why don't you try painting with a bigger brush?
Don't confuse your lot or one lot with everything that may have been done in your county. You are dealing with titled real estate and you need to focus on the one plot(s) that you are interested in. What their title chain has been. What the state laws are. And you totally missed the concept of adverse possession, if Florida recognizes that the title to the underwater property may have been lost to the adjacent landowners.
By all means if you've got money to play with and don't mind the chance of losing it all, roll the dice. Otherwise, buy whatever you want from someone who has a clear recorded title and make sure you get title insurance on it, in case that title wasn't so good after all.
Taxes and appraisals? Can be wrong. They're only an opinion, they don't change title, as far as I know. Owning submerged property, or bottom rights, is not at all unusual in many parts of the US.