Most vehicle fuel tanks with sealed filler caps and flat-top construction have two vent locations in my limited experience, connecting to opposite sides or ends, and commonly opposite corners. Alternatives are to have a vapor dome, or curve the top of the tank with the vent at the highest point, or route a single vent line to a small header tank with it's own vent arrangement. Here's why flat top tanks benefit from two vents:
Imagine a tank almost completely filled with cool gasoline. If the port wing is a little bit low, the tank will also have a bit of a tilt. The port vent connection on the tank will be submerged in gas, while the starboard one is above the liquid level. Now warm the fuel slightly, so it expands a bit. If there were only one vent on the port side, then the warming and expanding liquid gasoline would be pushed out the vent, even though there is some room left in the tank. With two vents, there is always some room to expand at one of the vent locations, unless the tank is 100% full. In that case, with luck, the manifold will route any displaced gasoline into another tank, rather than on the ground.
Wasn't "gas on the ground" one of the 3 useless things, along with altitude above you, and runway behind you?
The pictures are a big help. I've got to get the CD's as soon as my membership arrives.
Regarding the tank internal bracing - lots of rivets do indicate lots of internal structure. If the apparant thinness of the outer surface is an indication, the internal bracing may also be equally thin. That works fine for holding fuel in, but not resisting external pressure. Any internal structure is certain to be heavier than foil, but not much more. I hope to find some contemporary tank construction for simple analysis. Until we know for sure how the tanks were made, it's only conjecture how they would respond to submersion. With vent lines out the bottom, trapping the air inside them, intact tanks would offer a LOT of excess buoyancy, even if they partially crushed until pressure was equalized. In that case they would be metal balloons. However, if the plane tried to float nose-down, as Ric suggested, then the vent lines exiting from the belly behind them would be "above" the tanks. If the tanks crushed, then they would "deflate" and quickly lose all buoyancy as the air escaped through the vent line. If they stayed "inflated" due to strong internal bracing as Gary suggests, then they would continued to provide a lot of excess buoyancy even if vented to the open air.
If the plane was banged around on the reef, then anything might have happened, but damage would be certain.