It's less of an issue on a boat where a lot of it is non conductive with a GRP hull and consoles, but conventional wisdom in most applications is to put the fuse as close as possible to the battery and on the live feed wire.
This is because a significant proportion of problems in typical automotive applications are down to chafing of the live feed rather than a fault with the actual device. As an example we once had a vehicle in our workshops which had a small fire in the dash where somebody had installed a VHF radio, the radio had 25A fuses on both wires in its power lead but they were just behind the radio, so when the positive wire chafed through on the sharp edge it was resting on, the rest of the fairly chunky wire was a direct unfused link to an 800A battery... not ideal! Fortunately it just did a bit of welding for a couple of minutes and then burned out so there was not too much damage but things like that are just as likely to burn something out completely.
You can put a fuse on the earth lead and it doesn't hurt but ...
a) it doesn't help in the above scenario so you'll still want one on the live side somewhere
b) there is no benefit in it protecting against a chafe to earth as the metal chassis of most substantial machines is earthed and therefore even if a wire does chafe through on the earth side no current would flow, and
c) there's no benefit to the device over putting in on the live feed
A Boat is a hole in the water, surrounded by fibreglass, into which you throw money...
Sent from my Computer, using a keyboard and mouse