If I'm
selling a boat, I would say "WOT 34 MPH". "30 knots per hour" would/could
make
someone think the boat is slower if they didn't know.

Faster boats are frequently logged in MPH. A fair number of mfgrs sell boats
with speedometers that read in MPH.
That's only "deceptive" if the boat doesn't actually travel that fast.
Raymarine and other major electronics mfgrs build some pretty sophisticated and
expensive DGPS equipment that is strictly for marine use, and they build in an
option for the owner to
show speeds in either MPH or knots. Somebody needs to get 'hold of Raymarine
and insist they knock off selling that "deceptive" DGPS equipment that can be
set to show speeds in MPH.

On the other hand, trawlers are almost always described as turning speeds
measured in knots.

Deception would be recording a speed in MPH and then representing that it was
knots. A speed measured in MPH and clearly labeled as such may not suit the
personal preferences of some, but is hardly "deceptive" unless the reader or
listener doesn't understand the difference between one standard and the other.

I'm going to an Armadillo barbecue hosted by Billy Schumacher this Saturday.
(There will be a couple of classic hydros, including Miss Bud, on hand).
Billy's traveled one heck of a lot faster on (or slightly over) the water than
99.9999% of us ever will. I'll ask him whether unlimited hydros measure speed
in knots or MPH.
I know that both terms are fairly interchangeable when discussing aircraft, it
will be fun to see what the protocol is for hydros.