wow, these are some really bad ideas... no offense, they just won't work
Refrigerators work by compressing a gas into a liquid (low to high pressure) and then releasing the high pressure liquid into a low pressure gas again. The high to low pressure transition causes the refrigerant to absorbs a large amount of energy from the environment surrounding the enclosed refrigerant as it boils from a liquid to a gas. This is the part that makes the inside of your frig cold. the low pressure gas is then recompressed and runs through the coils on the back of your frig to release the heat that was just absorbed from the inside. Your home and car A/C units work the same way.
the only way to prevent the condensation from occuring is by starting the refrigeration in an extremely low relative humidity environment. cold air does not hold as much h2o as hot air. 50% humidity at 25*C will be +100% humidity at 5*C resulting in condensation.
The only way a plastic bag would help you is if there is a vacuum inside of it, or no humidity. the air inside the bag will be gettting as cold as the air outside of the bag... so you really havent done anything.
Do not submerge your computer in pure H2O because there is probably grease and dirt a)inside of your tub and b) on your computer which will thus make it conduct electricity. I am also fairly certain that olive oil conducts electricty, not sure off the top of my head.
still think mechanical engineers don't have anything to do with computers? go ahead and throw your computer in the bath tub and see what happens