Hi Geoff - I've actually used a three layer process. First sound deadener That's the silver coloured product on one side with black bitumenised material on the other. Then waterproof underlay which is self adhesive. Thirdly I used (and you are absolutely correct) marine carpet. Two types of marine carpet - thin for the cabin sides, foot well walls and cabin generally and a heavy ribbed marine carpet for the flat floor areas which is not glued in but removeable for cleaning and drying. The supplied military mats go back in over the top just to keep the originality supporting functionality. It makes the 6x6 @ 90 kph a comfortable vehicle to be in and conversations can be had without the need to raise your voice. I cut around all of the inspection plates so that they are removeable to provide access to fuel lines, transmission etc. so the installation is functional. Mud and #$ap on the carpet removes with a bit of effort which is much easier than I had first imagined by allowing it to dry brushing and a vac. The floor, when completing a river crossing, (remove the loose carpet) drains well and I have a little 12v wet vac I use to get the 'tidal' areas free of water. So far no odours remain from the water crossings when I have got the excess water out. It is worth the time to pull the seats apart whilst you have them out, clean out the gunk from the tracks and repaint. I then sprayed a graphite onto the seat tracks and moveable mechanism before re-installation and the cabin is like new, my WO mate says 'better than new'. The idea for this came from "Robert 42" from this forum.
Hope this helps - let me know if you want more information. ps (I used the access hatch to make a template to cut the carpet and underlay and this sped up the process a treat)
STDDIVER