That has more to do with mass than the planes (although you could try altering the coefficients for those), I believe.
Mass has a lot to do with how much "inertia" a vessel will have. This means a very heavy boat will resist levelling at high speed, when combine with buoyancy. Balancing mass and buoyancy has much to do with a stable boat.
Don't forget the game has an absolute limit to just how deep you can go. Imagine the world's oceans being like a hollow ball, with a blanket floating on top of and below the surface representing dry land and sea floor.
The thickness of the ball's skin represents the total depth of the ocean. This total depth is 350 meters. Go deeper than that and you'll get a CTD.
300 meters is the magic number. Depth charges cannot go past this depth, in the stock game.
