Prop guard, get one of those cheap replacement BBQ grills and bend to shape.
Have a hook on the top of each float. Use a coat hanger bent into a diamond shape, lifted and lowered by a servo. Catch the hook on the float and pull to shore. Might not be possible if to fat away to see. To launch, drop, let go of the bobbers, you could use a glider tow release setup.
I am sure as you build it, you will figure out the engineering issues as they come up. Post a couple pics when you get it all setup and working.
Buzz.