The story goes, the original developer "thought" he could force the issue and pay someone off and get a canal dug of lock installed.
Word got out he was attempting to bribe someone and they made sure it would never happen.
So, unfortunately, you cannot access sale water from the fresh water in Rotonda.
South Gulf Cove (the subdivision to the east) has a lock system.
Hope this helps - go to http://TheLotGuy.com and you can get a lot more information about lots and land in the area.
Need to get over to South Gulf Cove to get gulf access and then 5x the cost of lots and a two hour ride. Lots of hype about the gulf access but as a practical matter is is not practical at all and it is expensive.