403 feet down is not far to go. If they were at top surface speed and did a crash dive and either had the planes jammed by a depth charge or took water in at the bow, they'd almost certainly plant themselves as they appear to have done.
They had been under attack, so they may have been in an emergency dive, in unknown depths....................bad decision..................
403 ft may not be an impossible technical dive, but all technical dives carry an inherent risk. Add the fact that these are not ‘diver-friendly’ waters and the goal would be to do a close-inspection of a 70 year old wreck (possibly entering it). Short of a navy salvage dive team there are few who would have the resources to do it safely, and they won’t do it for reasons of international treaty. If anyone else attempted a large-scale dive the German government would have it shut down.