Firstly are you sure its 36v that is an unusual
battery voltage for a
boat. 12, or 24v is much more common.
The main house fuse (if one is fitted) should be close to
batteries.
The most common type is like this:
There are some fuse holders that fit directly on the battery terminals, but they are rare and reasonably new.
Don't overlook the fact it could be a problem with the main battery switch or one of the connections along the way.