id have to agree with jmj... initially i thought becasue of the fog he may not see the sub but after thinking about it, (im sure the devs didnt think this far into it but...)
with the sub being IN the fog, and the plane flying over the layer id think the plane would definitly be able to make out a surfaced boat.
because it IS like flying. the sub sitting in the soup may only be able to see a short distance in the fog, but the view of the fog from well above wont look like much at all from the plane because its being viewed at such an angle the visibility through the fog (from a plane) is such that you can see practically right through it, thus seeing the sub.
a layer of fog from aloft to a pilot wont look like much at all, but when you descend into that same layer and look through it at a 'longer angle' the visibility can get so low it could inhibit landing completely.
last thing to consider, it the games credit is that most of the aircraft nagging us are low wing planes thus making tons of blind spots for the pilots to look out. again, i dont think the computer cares about that really either.
and i run reduced a/c attack mod, the few planes that do come nag me, head straight for the boat, no questions asked
