Dips***t. I know the difference, I was goofing see the
. Argh.
I've seen them break. They get moisture in them, they go through freeze & thaw cycles, and you really have no idea a blocks history, and getting back to your wiki link I'm confident saying most people cannot identify the grade/composition of the concrete used in any given block.
You already said the marina stacks them, and that is far different then using them under a car where stacking them in a proper way is not very possible - you can't get a car all that high. Plus no one is probably lying under the boat, working under the boat and so forth. Apples & oranges. You appear to say it is ok to use cement blocks under a car and I'm telling you from experience it is not a good idea.