Water or mercury aside, is your engine compartment firewall sealed from the cockpit?  I know that the cockpit is a low pressure area, but with the positive lower cowl pressure the difference is significant enough to force engine cooling air into the cockpit through any opening however minor. 

 

The secondary issue is the use of cockpit air as a secondary static source - as you can see, there are differences if vents are open or closed and differences related to speed.  For safety sake, if I use the cockpit air and an alternate static, I expect the altimeter to read 400 feet high and the airspeed indicator to read 30 KIAS faster at approach speeds.  This is to avoid meeting the ground before I should and not stalling the aircraft on approach - Arrrggghh, even the AOA looks at static air.

 

I think those old leaky spam cans did not have this magnitude of pressure difference when your CFI recommended smashing the VSI glass to provide the emergency static air when the static port froze over or filled with water.  However, using alternate static air in the Skymaster (flown in a prior life), resulted in speed and altimeter reading differences that were documented in the POH (200 feet on the altimeter, I don't remember the speed change).

 

What's in your POH?

 

Did you include this determination in your flight testing phase?

 

Does anyone care?