I haven't bought one because they don't work, endless ice supply
needed.

20 pounds of ice @ 144 btus heat of fusion per pound is 2,880 btus.
Raising the temperature of cold water would provide a bit more cooling.
So it's about a 3000 btu air conditioner for one hour, assuming its
heat exchanger setup had enough throughput to melt all the ice in one
hour. An AC air conditioner, for example the "Marine Air Portable
dockside AC unit effectively cools cabins on boats 20'-30'; 6,720
btu" so it's half the cooling of a *small* air conditioner (ignoring
dehumidifying effects, we'll assume it's 100% humidity on a stifling
day in Panama). Pretty lame, I'd suggest putting the ice in bags and
sittin' on 'em.

Stretching it to last 10 hours would require 200 pounds of ice, or an
output drop to 300 btus an hour, typical cooling capacity of a 12 volt
Adler Barber Cold Machine air cooled reefer setup goin' full blast.
Not much if you're trying to cool a boat, test by opening up your
reefer and blowing a fan on it.

Beware, they also make evaporative type "Swamp Coolers", which *do*
work great in low humidity areas like the desert. My Dad had one with
an intake cowl on it (no electrical wires) that fit in a partially
unrolled window on his '69 Jeep Wagoneer; you had to pull a cord
periodically to rewet the evaporative pad. At 60mph, pullin' that cord
resulted in a blast of cold air! Also had to refill it often if it was
ragin' hot and dry.

The only 12 volt AC I've heard of that might work is one that sucks up
water 10 feet below your boat, hopefully below a thermocline, and runs
air through a heat exchanger onboard. Pricey though, cupronickel
tubing.