Look at using a float switch. Place the switch in a protected location where wind and waves won't cause it to bob up and down. You'll have to do some local engineering install it.

A different choice is an on demand switch system. It uses 2 switches, the lower one turns it on and the upper one turns it off. You do need a latching relay to hold it on.

Sure. How much money you got? W. W. Grainger will carry them. Which model depends on some specs that you don't list.

I want to be able to start a well pump filling our fishing pond in a remote location and have the timer shut it off after a delay of 1-5 days; without my having to make another long drive to stop the pump. Does anything like that exist?