I would not be too quick to dismiss Swordfish and Sea Gladiators as old biplane aircraft. The Germans and Italians did so and deeply regreted doing so. Most of the German heavy ships at the time also had biplanes for recon and range finding.

Can you give an example of the type of seaplane or flying boat you had in mind for the submarines ? I would guess it would have to be big, multi-engine, and very sturdy. The Germans had the Blohm Und Voss Bv-222 and Bv-238 but they were huge and would cost a fortune to build 300. The Bv-138 was a good design but could not carry four torpedos, unless they were of reduced size.

__________________
I need a bailout of only $500,000