He is going to have to cut a hole in that bulkhead to get to the under side of those bolts. Going to have to be a pretty good size hole to get your whole hand through there.
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
Actually, the rear beam bolts on my boat are aft of the first bulkhead, so I was able to at least feel them by taking off the aft deck plate. Not sure if they changed the build on later boats though.