I think this is a nifty idea. While at a classic car show, I picked up a square of self-adhesive soundproofing material for a couple of quid, and am using it to add weight to the tugboat.
What I'm hoping is that the dense, bitumin-like material will stop any resonance from the plastic hull and deck. It's flexible so not difficult to form around the curves of the hull. I don't really need the shiny, heat resistant side other than to stop things sticking to the black layer, but it looks pretty.
Covering the bottom of the hull and underside of the deck near the motor added valuable weight, but I still need lead to bring the waterline up the hull a bit.
I'm pretty sure the noise killing has worked a bit. The drive system gives a nice, deep "thrum" sound now which is pretty realistic. There's not squealing or odd vibration. I'll be doing this again.