I've seen muffler-like attachments that connect to the exhaust.

I have one for my noisy Ridgid - makes a little difference; not much.
It seems to me that if you built a box out of 1/4" pegboard, and lined
it with carpet, the air could still escape but much of the noise would
be muffled. You could even line the top of the box with egg-crate foam
or acoustical tiles or something, and since this relatively
high-frequency noise is fairly directional, extra insulation in the top
should help, especially if the vac is down in a boat. No clue about dB
levels.
Another thing to consider is a longer hose, so you can "hide" the vac
where it might be more isolated/insulated. Woodcraft, Lee Valley, and
others have longer, better hoses, i.e.
http://www.woodcraft.com/family.aspx?familyid=914
Regarding overheating, I wouldn't worry too much. I've run my Ridgid
for probably an hour straight, hooked to my plunge router that has a
restricted (1") intake, with the filter increasingly caked with dust.
That seems like a recipe for overheating, and though the motor felt hot
after this, it wasn't smoking/burning/extremely hot.
Of course, the best option would be to buy a Fein - the Turbo I ($205
at Amazon with $25-off sale) advertises 63dB, comes with a 15ft hose,
and I've never heard anything bad about them (other than the price).
However, considering I use my shopvac more than any other single power
tool, it would have made sense to buy a really good one. When the
Ridgid dies...
Good luck,
Andy