The first thing you need in order to refuel is a fuel connector.


The next thing you need is a source of fuel, right now that means the fuel tank mounted on the docks at Portus (there is also one on the runway). In order to utilize this fuel source you need to have at least 1 fuel connector item in your inventory (the yellow one). These can be purchased at certain buy shacks and spawn in your inventory in sandbox mode.
To make the connection, use interaction-mode and right-click the fuel tank connector on the dock.
Once you have activated the options you will be given a choice of which fuel connector to activate depending on what you have in your inventory.
After an option has been chosen you will enter setup mode for the specific connection type. During setup you will see a preview of the cable in either red or green. Green indicates that the connection is valid, red indicates that the connection is obstructed in some way.
Note: At any time you can cancel setup with right-click.
Once you have done that you can connect the cable to the fuel connector on your boat with interaction mode and left-click.
Note: If a connection is invalid the fuel cable will not appear. This can happen if the cable is too long or physically obstructed. If a valid connection is physically interrupted the cable will be lost and fueling will stop.

Once this is complete fueling will being. Fuel will be transferred in the direction that the connection is established (Fuel Tank -> Boat vs Boat -> Fuel tank). Now you can make any connection between fuel connectors, even boat to boat for refueling operations such as a tanker.
Note: In campaign, fuel transfers cost shells when fueling at the dock

When you are done refueling you can remove the fuel cable and it will be placed back into your inventory.

The primary way to attach a fuel connector is to put it directly on your fuel tank. When a connection is made directly to the fuel tank, that fuel tank will be used exclusively for the fuel transfer.

Fuel connectors can also be attached arbitrarily anywhere on the vessel (not directly to a fuel tank). When attached in this manner the fuel transfer will automatically pick the fuel tank with the least amount of fuel to either consume or fill. Once that tank is empty or full it will select the next tank in the same manner until all tanks are empty or fuel depending on the type of fuel transfer.

If your vessel has fuel groups configured. Arbitrarily placed fuel connectors will link to configured fuel group behavior which will essentially be the opposite of how the vessel uses fuel so that fuel tanks are filled evenly (or as configured). This is mostly useful for planes and technically allows you to refuel mid air (but that is likely very difficult as it would require a Nautikin standing on the aircraft to make the connection).

The fuel can can be used to make ad-hoc transfers of fuel from one fuel connector to another. You can also store filled fuel cans. Fuel cans are sold at various buy shacks. If you have a full or empty fuel can in your inventory an extra option will appear when interacting with fuel connectors.

Note: Dropped fuel cans that are full will explode if damaged.