Probably not very well. Forcing the water to make a ring around the perimeter of the exhaust piping creates a jacket of cooling material, much like I'd imagine Preparation H must do similar in fashion, makes a hot hole more enjoyable and less irritable.
Having poor cooling and hot spots makes for bad mojo on a boat. We had someone make their own water mixer rather than use the one that was supplied with the engine once, they wanted a 6" mixer rather than the 5" supplied and simply use a hump hose to increase the diameter at the point of entry. The resulting design, rather than having the proper number of correct sized holes to force the introduction of water into becoming a jacket to protect the exhaust, it looked like someone took a triangular can opener - the kind you might find for those large cans of V-8 or pineapple juice at your local grocer - and jammed it through the metal to create outlets. This piss poor design ultimately made the marine exhaust hose fail after a paltry 43 hours of operation, the interior sleeve separated from the exterior and it swelled up like a blister on the inside. Not to mention the smell which was noticed at the time of the sea trial and when I pulled it apart we had a long conversation about warranties and how downright foolish the owner and builder were installing this particular setup. After being called down to fix the situation, since the owner didn't want to cure the issue by fixing the mixer itself, we introduced a device that disrupted the flow of water an made it swirl around the pipe. That seemed to do the trick - although not optimal in our eyes.
Cooling is crucial for any exhaust, and when you're messing around with water it is truly paramount.
