Roger
Your theory about slowly replacing the fuses seems workable to me
although the circuits should be wired in a manner that enables you to
knnow the total current rating for the cicuit already. If you can
isolate all the gear on a circuit you should be able to calculate the
total circuit load. If you have quite a few appliances connected to one
circuit (not ideal) you then need to calculate what you may have
running at the same time and size your fuses/breakers to suit. this is
called a maximum demand calculation if you want to try and google more
info. I appreciate you are playing with someone elses bodgey work and
a rewire may not be practical though. Can't you install breakers in a
water resistant box though even if they are inappropriately placed
under the companionway. This is just my five cents worth as an
electrician/boater and i do not mean to sound like an Aussie smart arse.