Here is post where it was discussed before:
http://www.subsim.com/phpBB/viewtopic.php?p=278077
Its been brought up a bunch of times....I think the main problem is SH3 doesn't model bilge pumps, ballast, or trim correctly....it would be alot more interesting to have a negatively bouyant boat if you could manually attempt to fine tune it...and compensate with bouyancy during damage, flooding, etc......otherwise it has to be like a hummingbird to be automatic like everything else in the sim....
I have a feeling once you start taking damage...after removing the hummingbird effect...the game will become irritating because in reality you only can pump air into your ballast like 4 times total (the amount of times the developers allow you to use your compressor based on the default presets) to compensate for negative effects related to damage.....there needs to be more refined control of at least the ballast in order to compensate once your boat begins to take on water...I can see a scenerio where you might be deep and sink even though you are going at a fast speed but cant maintain depth because the ballast doesnt aid like it should.....