I put a layer of closed cell foam, (blueboard insulation) in the floor of mine, in between the ribs, i then went to the local metal yard and bought a sheet of aluminum and cut to fit my floor, i then screwed it to the ribs in the boat and layed marine carpet over it. this was a 1440 flatbottom, cost me right at 100 bucks for everything, made it look better and was much quiter with very little weight gain. for the underside I used a fairly thick layer of gelcoat paint for speedboats, to me it was a little quiter when the water hit it. If you have the floor, that will help deaden some of the echo from the water slapping it.
P.S. my center seat was removed so it made it a little easier to lay the floor down, but if you still have a center seat, it only requires one more cut.