You need both, the mixer injects water with the exh. to cool the gases so they may be piped off the boat in a rubber hose. The water lift muffler allows the exh to push the water up above the discharge point so that it may flow out with the exh.
unfortunatly when you mix stainless, hot salt water
, and exh you don't get much life out of them. Cast iron actually lasts longer in most cases. You can make your own (if your handy) out of bronze pipe, or just bite the bullet and buy another one of these.