As has been mentioned transferring fuel at sea is a nightmare. I love jerricans but they are big heavy and made of metal that when bouncing around breaks things. Therefore I carry all my spare fuel in 5 litre plastic cans. They are easy to carry, easy to store and when bouncing around don't have the same impact as a full jerrycan. Reasonably easy to refuel in a rough sea as well. They probably have downsides.
Oh and they are totally legal to use in Petrol stations. Suprisingly 20 litre metal Jerricans are not in Hampshire. But 10 litre ones are