A few comments

A few comments on synopsis ideas number 1 and 3: these could, besides having the potential of becoming interesting movies in their own right, also be a great way to make a sequel if idea number 2 will turn out to be the best way to go.

A little concern about a rescue-mission-based synopsis: It was more of a rule than an exception that u-boots did sank without anyone knowing the reason for sure (due to restricted means of communication) - and later, as far as i know, it was concerned to be a necessary sacrifice: if a boat sank, was shot asunder or had major malfunctions, noone would look for them. But, on the other hand, if the goal with the mission is to by scientific evidence proove the supremacy of the 'aryans' and therefore in some respect justify the ideology which would have world-wide political concequences, they'd probably put more effort into succeeding and investigating any failed attempts including missing and/or derelict vessels + crew.