Good idea, erps.
The only advantage to my five gallon buckets is that they are "light" enough to handle. Engine blocks, if cast iron, are nice once they are on the bottom. In fact, there are lots of things that will work nicely once on the bottom. The fun part comes about when you trying to get your weight from your vehicle to a boat, then over the side of the boat, and right where you want it. Two five gallon buckets of cement are about all you are going to want in the stern of your basic row boat. The only good way I can practically envision getting something heavier out there is to laod it onto a raft, off a really well built dock. Then all you have to do is roll it over the side when in position. Do not underestimate the effort to lift a good weight engine block over the gunwale of a decent size boat. If you load the engine block in the row boat I would recommend one of two things: 1. Video tape the part where you lift it over the side of the rowboat out on the water. 2. Row out into position in your friend's row boat and sink the boat with the engine block in it. He hardly used that boat anyway, you are now an artificial reef and not a mooring, and you are only tied up to it for recreational diving purposes. Seems to cover all the bases. Of course you will need to display the proper dive flag while moored.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.