Fuel notes:
I have done several deliveries over the decades where we had some extra five gal. plastic containers on deck. Lashed them to lifelines with some padding on the lines to prevent chafe. Have them very near, if possible, to the deck fill fitting. Use a "rattle syphon" to transfer from from the container to the fill access. No spills. Works well.
And then, when considering the maximum that your boat's tank holds, in regard to your ocean motoring range....... always subtract about 3 to 5 gallons from the total to allow for the need to have some fuel in the bottom part of the tank.
When the boat is rolling around it's too easy for a very low fuel level to allow a slug of air to to enter the line and kill the engine. (Bleeding the injection system while the boat rolls wildly back n forth is no darned fun at all.)
As far as extra drinking water, I have had larger plastic tanks built that much better fit the areas designated for the original install. We took the boat from two tanks of 23 gallons each to two tanks holding about 62 gallons total. Having more than one tank and then using a manifold & valves to direct the delivery always lets us know when one is empty, and valving over to the other means it's time to consider resupply.

Regards,
Loren