Rent a robotic total station with a data collector. Figure out where exactly you want the coarse, set up 2 known points in relation to the coarse on land. This could be done with the total station and a couple of buoys. The best thing I have seen was a buoy with a threaded male insert glued to the top, all prisms I have used have 5/8" 11 thread couplings. You will need a 360 degree prism. If you have this setup, you could have a guy in the boat holding the data collector, advising swimmers/ divers how far to move each buoy and in what direction. After you get it set, it should be record tolerance. The total stations are extremely accurate. One bit of advise is to let the total station see, clear line of site, the prism all the time but if it looses it, it's no big deal but costs time. Best if you have someone familiar with these systems. GPS system is another, maybe faster way to do it. Most sophisticated contractors have these systems and surveyors. It really should not be that expensive to do.