I wonder if it is
hydrilla. Sounds like the name of a water monster in a movie, but
hydrilla is for real. Bad stuff.
Here is a link to some information on invasive species in Florida. At the bottom of the page, you will see a list of the top 10. If you want to, you probably can look them up by name and maybe even see some scary pictures.
Hydrilla is #1.....
And after you gather all that info, you can be a veritable font/fount
(I never know which one it is when spelled) of information at your next driveway party....or everybody will just think you are Cliff Clavin.
Anyway, if you really want to know what the invasive species might be, this link will get you started:
Invasive Plant Management
Marian Boomer