If this is a fuse mounted on the battery terminal, it doesn't make any difference which terminal it's on. Voltage is a potential difference
. Ground isn't a relevant term unless you have a wire connected to it.
Farther down the wire, fuses
for secondary devices on the hot side make sense. Reasons for this are on cars every bit of metal around you is ground, if the wire upstream of the fuse chafes through it's a short. Metal boats same deal, entire engine
on your boat same deal, wet bilge
full of salt water
If you disconnect the negative battery terminal from the boat, there is no reference. You can connect the positive terminal to anything, say your bonding system, the plus terminal is now at water
(ground) potential and the negative terminal, is -12 volts, connected to nothing, doing nothing.