I'm trying to politely say that this doesn't really work. Unless the subject is tiny. There's a good one of a Sherman tank sunk off of a pacific beach. But there, the tank is 1/72, and the turret top is at about water level.
So the whole thing is maybe 4x4x6, and mostly model not gel.
Any bubbles between layers will ruin it. There would be multiple layers per inch, resulting in dozens of layers. The chances of getting ALL of those right are next to nil.
It gets expensive too. And if it gets muffed, both the expensive resin AND the model go in the can.
Much better to suspend, or mount the model in a clear plastic box, and paint the back black or dark blue or other effect. Make a top with color and waves.
The last thing. The calculated crush depth of a U boat is about three times it's length. So set the thing relatively near the surface.