First off, the largest gun on a pt boat is a 40mm bofors.
You have to adjust how much lead you're giving your target (aim where it will be, not where its at. Just in case you don't know
) based on how fast its going, and how steep its diving, or climbing. Always aim infront of the nose.
You can only hit something out to 1.5k MAX, even though the bullets will travel further than that.
When people strafe pt boats, they generally come in at pretty shallow angles (to give themselves the longest firing period possible?), wich makes aiming pretty strait forward. Concentrate on the planes that are attacking YOU. They are flying toward you, so you just have to point your guns at him, and cut loose.
I'm in no way an expert at AA gunning, but this has always worked for me.