Tuned Pipes are the best to get more performance out of your boat. The trick is to choose a header (manifold) that fits your engine and inside your hull. Next is choosing the pipe itself. They both come in lots of shapes and sizes. Mostly it depends on the engine size. Lastly, decide if you need muffled or not. A muffler will add about 4" to the overall length of the pipe.

Muffled Tuned Pipe and
Header
assembly
Header or Manifold
Pipe extender by Hobbico
Pressure fitting
Reusable clamps work great instead of zip ties

Tuned pipes come in many different shapes and sizes.
We have had good luck with MAC pipes and headers. The offer pipes in muffled and non-muffled versions for a wide variety of engine sizes. Be sure to match port the Manifold with the exhaust port on the engine (see Tips).
Make sure to use a good Silicone Connectors (couplers) with pinch clamps or zip ties to hold the header and pipe together. Prather makes really good ones in the three main diameters!