I've read the manual and I'm pretty sure you can't refuel ships at sea. I think the reasoning behind this is the relatively short length of the scenarios.
It is possible to run out of fuel so the only way to replenish must be to visit a port.
From the manual (Page 24 Section 10.0)
"It is entirely possible to run out of
fuel in CAW and be forced to steam
at 5 knots. Steaming at flank speed uses a large amount fuel, so
it is important not steam at high speed unless it is necessary.
A TG that has reached a destination at sea is assumed for game
purposes to be steaming on station at 20 knots, or its maximum speed,
whichever is slowest.
I saw generals create imaginary "masses of manoeuvre" with a crayon and dispose of enemy concentrations, that were on the ground and on the map, with an eraser. Who was I to criticise them, hero as I was of a hundred "Chinagraph wars" of make-believe?