With the stock game, the easiest thing to do is to use
Webster's Better Air Patrols mod. It turns down the frequency, but increases the planes accuracy, so it's more important not to be seen. Use the SubSim download link, since Filefront is no more. One thing to remember about the airplanes is that they have these super-radio sets in their airplanes, and can fone home and tell all their buddies about your submarine traveling on the surface at 10 knots, course 276 142 nautical miles off of Kii Suido. So don't let the first one see you. If he does, dive below 100 foot, and then change course. If it's late in the day, just stay down until dark, then get back up top and continue on your way.
Edit: Corrected the link - when using keyboard shortcuts, one does have to be careful with how long they press the keys...
