Yep. The other part of it is that the longer the pipe, the more time the exhaust gas has to cool, and thereby the more tendency it will have to fall. When it contacts the water (which will presumably be cooler than the gas), it will (in most cases) become sediment, instead of exhaust gas/smoke, and settle to the bottom of the water naturally. Or at least part of it will. I think.

*gets his engineering book*