Large Object Warning Fish Finder?

I am searching for a Fish finder/Sonar with a very specific feature. I live on a large lake in Orlando and my boat house is next to my swim area, I would like to be able to turn on the device when my children are swimming and have it alert us when something large enough to pose a danger enters the area such as a large alligator, can you recommend the appropriate device?

Thank You.

I am afraid there is no product that I am aware of that could reliably be used. A sounder will only see objects within the transducer beam. A sonar, be it search light or Omni could cost $15,000 and up. It would also be limited to having the entire beam width in the water. Once the upper portion hits the surface your range is limited, so gators on top would not appear until quite close by.

Perhaps another moderator or forum user might chime in with some other ideas or thought.