My mechanic swears by freeze spray you can buy in a can. I think you are supposed to spray it on before you break off the bolt but it works by instantly shrinking the bolt a fraction thereby breaking the rust bond.
The accepted method of breaking rusty bolts free is to heat them up. There is a reason for this.
The expansion or contraction of metal due to heat or cold is proportional to the original size of the hole or the bolt. Since the hole is slightly bigger than the bolt, if the bolt and the backing plate are the same material and you spray the spray the whole thing down so everything is at the same temperature, the hole will shrink more than the bolt and just clamp down on it.
In this case the bolt is SS and the plate is AL. The linear rate of expansion for AL is ~4/3 that of SS. If you chill the plate and the bolt the hole will contract 4/3 faster than the bolt and probably make things worse.
For a rusty steel nut and bolt(like on a car), using freeze spray can crush the rust and free things up. For a catamaran's beam bolts the "rust" is probably more like a super thick white goo. Chilling can compress the goo into the threads and make things worse.
Sorry
