Unless you can find enough unused space below decks to squeeze in a
fuel bladder, you're stuck with carrying extra jerrycans of
fuel on deck. It may not look real pretty, but you'll fit in with all the other cruising boats anchored around you.
As far as
fuel capacity goes, unless you're crossing oceans, you'll always be close enough to a marina/fuel dock to top off your tank when you need to.
I always carry 2 full jerrycans of diesel when I'm cruising. When I've almost emptied the last one topping off my main tank, that's when I think about a
fuel dock visit. Using this approach, I can't recall the last time I pulled into a
fuel dock to add
fuel to my main tank. I just dink in and get both jugs filled.
Your guestion was a good one, especially as sailboaters probably spend 50% of their time cruising on the "iron jenny".