FWIW... that last can put the boat cooling project in perspective.

Let's say the cabin was small enough to cool with a 5000BTU AC.

That's 5/12 of a ton, so it would take 5/12 ton of water ice to provide
the same cooling capacity (ignoring the other thing about extra calories
derived from raising the temperature of the water)

Airgas corp. says dry ice has three times the cooling capacity per pound
as does water ice.

So now we only require 5/36 ton of dry ice to cool that cabin for 24
hours.

So that's what... about 278 pounds!

LLoyd G