Dump truck in mine

[DetectorGuy]

If these systems aren't sealed, our shop would be very busy replacing motors. 

When we pulled a car out of a local lake that had been down on the bottom for 8 years it still had all those components in it.  When the air bags started lifting the car off the bottom, a fuel line must have cracked causing fuel to shoot to the surface.
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I am not a mechanic (and I hate working on vehicles) but the following are my reasons I believe the liquids could leak in certain underwater conditions.
I might have mis-spoke on the possibility of liquids leaking from a submerged vehicle...
The oil as far as I know is a pressurized oil system to a point, as the valve train is lubricated by splashing oil on the valve stems and lifters. This oil that splashes is fed into the valve covers through oil galleys in the heads. The valve covers are then vented using PCV valves. It would need to be in ideal conditions for all the oil from the engine (which is lighter than water) to rise up through the oil galleys into the valve cover and then out through the valve covers.
The fuel also is a vented system as the top of the fuel tank has a vent line to prevent vacuum in the tank as fuel is being consumed and also to prevent over expansion of the fuel tank on hot days. The cap on a fuel tank is obviously not an "O-Ring" tight seal. This would also need to have ideal conditions to make it leak on a car as these vents would need to be facing up to leak while under water.
The rear differential is vented also and even though this is 90 weight gear lube, It could evacuate through the vent.
The antifreeze is a pressurized system and I believe the radiator caps on most have a 14 PSI pressure cap before releasing antifreeze into the overflow reservoir. The antifreeze in the reservoir would be subject to leaking as this is just a plastic snap-cap.
The battery acid in modern batteries is sealed but the older batteries with the individual caps (or 3 gang caps) would leak.
The power steering and brake fluid should be sealed with no possibility of leakage with out damage to a line.
Sealed beer bottles will not leak at depth but while bringing them up from 110' they will force the contents through the sealed lid  :'(. (I tried this one)

I agree it would be hard to have anyone of these liquids drain completely but it would be hard for me to say that some liquid from most all of the systems hasn't left a vehicle when submerged