Yes. The MDT 8000 and MDT 8000XD are IP68 rated.  This means you can submerge the detector in the water up to 1.5m for up to 30 minutes.
The MDT 8000UXO is IP67 rated. This means you can submerge the detector in the water up to 1m for up to 30 minutes.

For MDT 8000 and MDT 8000XD:
This is dependent on the desirable target that you wish to detect. Lower frequencies have better sensitivity to high conductivity targets. The higher the frequencies, the more the detector becomes sensitive to low conductivity and small targets.

For example:

• If you wish to have better sensitivity towards silver coins, it is recommended to to use the lowest frequency (6.4kHz on the MDT 8000 and 4.8kHz on the MDT 8000XD).
• If you wish to have better sensitivity to small gold jewelry,it is recommended to use the highest frequency (18kHz). 
• For mid conductivity targets (relic hunters), 9kHz and 12kHz are recommended.

Both sensitivity and threshold affect the detection depth. The sensitivity affects the depth equally for small and large targets. The threshold slightly affects the detectable depth more for the smaller targets than the larger targets.

No. All modes are equal, there is no gain/loss in depth performance based off of the mode you are using; however, if you are in the All Metal mode, this allows you to run the sensitivity a little higher due to better audio feedback.

The Pin Point numbers represents the relative target signal strength on a scale of 0-99. The stronger the signal is, the larger the numbers are.

If you have rapidly changing ground mineralization and you are not aiming for the highest possible sensitivity, tracking can be used. The tracking only compensates for the ground mineralization. 

The warranty for MDT detectors is 18 months.
https://tarsacci.com/warranty-policy/

When you turn on your MDT detector, on the bottom of the LCD display, you will see the serial number of your unit only during the startup for a few seconds.

If the ground is mineralized,  the detector has to be ground balanced for maximum performance.  The ground balance must be done on areas free of any metals.

There are two situations in which it might be preferable to manually GB over automatically GB.

1. When deliberate slight ground unbalance (offset) is desirable.
2. When the ground is not mineralized and the unit can not automatically GB, the preset GB is desirable (560).

560-564 are good values when the detector can not be automatically ground balanced.

First, make sure that you have performed a proper and correct salt balance. If you are seeing this drastic change it means that your salinity balance is incorrect and must be salt balanced.

Salinity balance should be used when detecting in salty environments, alkalite soils, and to compensate for some hot rocks (For example in case of natural graphite).

No. Salinity Balance does not negatively affect detectable depth.

Black Sand Mode should be used in cases of very highly mineralized ground and black sand beaches (Magnetite). You may find situations to in your environment where you gain performance using Black Sand mode while not in a black sand environment.

No. You do not lose detectable depth while using Black Sand Mode.