Sounds like a job for the local harbormaster or marine
For what it's worth/just for fun, if I remembered the right formula, an 18 inch sphere full of water
would weigh about 113 pounds. Concrete weighs about 2.3 gm/cm^3 or 145 lbs./ft^3, but it effectively "loses" some of its weight in water, so 200 lbs. of concrete hung from a boat
into the water would theoretically have a downward pull of something like 113 lbs. Cinder blocks weigh something like 30 lbs. each and are about 25% lighter than conventional concrete, so the numbers would work
out a bit differently. Of course, the buoy and rode
would have some weight to be figured in -- maybe about 15 lbs of buoy and a pound of so a foot for a wire cable or light chain rode
, more for heavier chain.