Fay's Boat Yard in Gilford has high quality concrete blocks made up by someone who knows what they are doing. Weighing about 250-lbs, 24"x16"x16" with a 1" pvc tube installed within the block that is used for threading a line; price, about $50. They last underwater in Lake Winnipesaukee for many years .... like 50-plus years or some unknown, even longer length of time?
Fay's will load it onto your boat, and then you go drop it overboard, where you want it to be.
"Let's see ...... if we put it right here ..... our raft should totally annoy our pain-in-the-neck neighbor .....and then we'll still have our nice big view ..... yes, I do believe we should place it right here ........ heave-ho ......big splash ...... ho-ho-ho ....
"