Re Dynamat 'Extreme', the shiny stuff is aluminum. No rust worries. I would say it's water resistant, not wateroof. Heavy.
You may want to look at their Dynaliner. Fully waterproof. Lighter.
Dynaliner
I've used both in cars I've owned/built.
IMHO, I don't think either will reduce the engine noise much. It's a single cylinder engine in an open cockpit.
Hope I'm wrong.
Spent quite a bit of time messing with this stuff this weekend. My conclusion...If you want to quiet the cab down, here are my suggestions.
#1. Yamaha bed mat helps a little. The OEM mat is VERY thick and all that sheet metal seems to resonate noise. Ya need something to protect the bed anyways!
#2. Spray on bed liner. I already have the mat in mine, but I'm going to add the spray on liner as well. NOTE: When you're sitting in the cab, just tap your knuckles against the front of the bed...TWANG! For this reason, along with the inside of the bed, I'm going to have the front (outside) of the box done as well.
#3. Back window. Haven't received mine yet...but I'm assuming it will help?
Those are the easy ones. Next, and here is what I believe to be the most important. As I posted earlier, I shoved a bunch of towels in every opening between the back of the seats and the cab. This weekend I went one step further and cut up a pool floaty for between the bed and the seats....AND added towels/rags
everywhere that noise might come in. There are SOOO many areas that are completely open to the engine compartment...and have no noise barrier what so ever. For instance, pop the drivers seat off and you'll see that you are sitting on a big void that is open to the rear. The seat belt mounts between the seats. I could go on, but you get the point. My wife looked at and thought I was nuts

until we went for a short ride! You could absolutely notice the difference.
In short...IMO most of what we are hearing in the cab has very little to do with exhaust. I believe we need to find a way to better insulate the cab from the engine/trans area. Some of you may remember Folks were using insulated type covers for the Rhino for the same reasons. Once we can find a way to just fill the voids, then I think it would be a good idea to look at some of the insulating liners.
If it didn't look so stupid...we could call this the "Pool floaty mod!"
M-N-T