Scott,

what factors would make a commander decide to fight it out on the surface?

Malfunctioning equipment preventing the u-boot from diving properly? (That would make sense.) Small number of airplanes? (Even 1 airplane would be enough to report the position of the boot; I would think it would make more sense to dive and change course than to try to shoot it down.) Other factors?

In terms of being caught by surprise, are you talking about a situation in which the airplanes were already close enough to inflict deadly damage before the boot could submerge itself? Would this be caused by low-flying planes that couldn't be seen until they were already dangerously close to the boot?

I only mentioned the difficulty of trying to shoot down an airplane in the midst of a dive because I was envisioning a scenario in which the u-boot commander was thinking "OK, I'll dive, but until the boot is submerged, we'll shoot at the airplanes." Now I'm getting the impression that it was a choice of mutually exclusive options: either dive or fight it out on the surface. Right?