Good project, John.  After we talked about this, I walked along the marina thinking about what the issues might be.  One thought was that sharp objects might be down there.  Another is that, in places the blue-green algae stuff is showing up, and that should be avoided.
As an alternative, how about if a line was tied to each end of a kayak, about 100 ft long.  The line would have tape marks every 10 ft.   Two people would move the kayak back and forth between docks at the center of each slip.  The line handlers would not go out on fingers.  The kayak guy would have a calibrated pole, of course.  This might be pretty fast and minimum time would be spent learning where the measurement person was at.  The slip numbers obviously set a coordinate system.  I have 100 feet of floating polypropylene line.  Most people must have some of that yellow stuff.  BiMart for sure.  I could bring a kayak on Thursday.    Bob