Once ships have fuel. If they don't? I never noticed it. But we could have a barge like the one that filled up my cruise ship. Along with a pilot boat. And you could either call one using VHF or go to one. There can also be on shore fueling stations. If your in the middle of the ocean. (Though you should have a ship capable of going across an ocean with 1 tank of fuel) you could find your way to a oil rig that has some extra gas on board?
I think that it is fun. If fuel was simulated like the rest of the game in real time you would have to play for a few weeks on a large ship to be able to fuel it. I just don't know how this would do in a simulator since you don't want to always run out of gas but you want it to be possible. For smaller sea going vessels it would be nice to have to go into port but some larger ships would be unrealistic.