A fully SH3-compatible destroyer sim would be nice to go with SH3.
What I would even prefer is an addon with international submarines : English, French, Italian or US (if pacific is developped) submarines, as well as other types of german ones (XXIII, UA, why not a midget sub or a milk-cow?).
A way that could be nice, and in the engine scope of SH3 is a WWI oriented simulator....
About a boomer sim, I feel that the water model is too poor for that : it is barely as much detailed for a WWII hydrophone station, but it would very probably be unsuitable for modern systems in terms of noise propagation, thermal layer... And the underwater model is all, in a modern sub-sim.
Another point is that helicopter has replaced the flying boats as the main air threat of the submarine, and no chopper is implemented in SH3, which means that it is probably not designed to accept them.