I guess I've been lucky. I've broke a lot of the rules for saving and have not had any problems.

<Knock on Wood>
I've saved in the middle of depth charge attacks while submerged, while approaching convoys(submerged and surfaced), and near land. When I've loaded the save, it runs fine. I'm sure I haven't tried all the possibilities though.
I always try to save on the surface away from ships and land though to be on the safe side. With limited playing time, that is not always possible. A lot of times I am making an approach and almost in firing position when I have to go pick the kids up or something and I have to save where I'm at and resume another day.
I've probably jinxed myself now and am set up for a corrupt save. :rotfl: