sometimes those hard black deposits are sand/mud/dirt some of it is actually a form of rust. most of us just send straight lake water through our blocks, and we all know water and iron makes rust. hince why if you run straight water in your car all summer then drain it, it will be all brown and nasty most of the time. I know some ppl motor the block and manifold drains are locked and wont turn. but if your does and you do notice you get into some mud and sand. you can drain you block at the dock once cooled off. your bilge pump will get the water out. I have even started motor before with those open and flush as good as I can. It also never hurts to flush the block one or twice a year just to be on the safe side that a lot of that is not sitting in there that we don't know about.