I had double wall full-length headers with a 4-1/2" collector plus a 12" extension (collector about 18" long of 4-1/2 stainless) completely dry. I was asked not so nicely to leave the lake we were testing on, as the lake patrol had "numerous complaints of windows shaking". Tried 4-1/2 inch switchable mufflers which did help tremendously, but even open they necked down to about 3-1/2 inches and seemed to take 1-2 MPH off the boat.

I went in where the overflow bung was (right behind the flange) and drilled a 1/2 hole right there in the inner exhaust tube, and sprayed water in there. That made it bearable for wife and neighbors, but you could still hear it for MILES away. More throaty, less pop!

(Sorry about the dirt, boat was in storage for 9 years and in restoration now)
https://cimg8.ibsrv.net/gimg/www.off...8fc0203cb2.jpg

Well, I sent them an Email inquiring about the marine line and they said they are presently working on it. Hopefully they get stuff out soon.