The Dive BT just makes the boat settle slightly. I think it's supposed to simulate flooding the bow buoyancy tank or trim tanks
If you set MainBT to 0 and Dive BT to a value (I used 10000) the boat will settle slightly from the DiveBT, but will not dive. It appears to settle low enough for the planes to get a bite in the water.
If you swap values and set Dive BT to 0 and MainBT to 10000, the boat still dives. However there is a bit of a difference in how long it takes to get underwater, but it isn't a whole lot.
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"There are only two types of ships- submarines...... and targets" Unknown
"you wouldn't catch me on a ship that deliberately sinks itself"- comment to me from a surface sailor. 
System:
AMD 6300 3.5 GHz | 32GB DDR3 | SATA 300 320GB HD, SATA III 1TB HD, SATA III 1.TB HD | ASUS Sonar DS sound card
NVIDIA 1660 Super OC | Windows 10
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