Just a thought, but one possibility is that the ferry tank was empty.
Perhaps supporting that theory is that aircraft was not on floats, but floated after impact. I saw PilotDar comment that there would be 6000 lbs of fuel in that tank. At 6.8 lbs a gallon, I take that to be empty, sealed, space capable of holding 882 US gallons. Surrounded by sea water, that tank empty would provide buoyancy of some 7540 lbs. If the aircraft was out of fuel in the other tanks, and empty wing and tailfeather spaces were not opened, the total buoyant space may have exceeded the empty weight of the airplane. It does seem odd that the airplane would float with 6000 lbs of fuel in the ferry tanks.
On the other hand, a ditching with 6000 lbs of fuel behind you is going to be an awful smack into the water with no power to control the flare and touchdown.
I saw a report where a Navy swimmer put into the water observed the two crew strapped into their seats, and pulled the leg of one of them, without response. Let's say there is a 30 knot wind out there, and that the forced can be made into wind. Flown perfectly to a touchdown at stall, that would mean a waterspeed of 28 knots at touchdown. What we have is a touchdown, or a loss of control, that was sufficient to inflict fatal trauma on the crew, strapped into their seats.
It's not realistic to think this crew, or someone they relied on, didn't fill the tank with every drop it would take. The empty tank scenario, which I'm certainly not wedded to, would have to result in all of the fuel porting overboard. I can't imagine the crew wouldn't have noticed that. Do those ferry tanks have fuel guages built in, or is it time and power setting that is the only check on what is left in it?