And I flew light planes and assumed you were talking about Denver... who knew? I did wonder why sea water and Denver were together in the argument…
Anyway...
Fahrenheit temperature scale
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The Fahrenheit temperature scale became popular through its use on the first reliable, commercially-available, mercury-in-glass thermometers. Daniel Gabriel Fahrenheit manufactured such thermometers in Amsterdam from about 1717 until his death in 1736. The scale we know of as the Fahrenheit scale was the last of three scales he used.
As the zero point on his scale Fahrenheit chose the temperature of a bath of ice melting in a solution of common salt, a standard 18th century way of getting a low temperature in the laboratory (and in the kitchen, as in an old-fashioned ice cream churn). He set 32 degrees as the temperature of ice melting in water. For a consistent, reproducible high point he chose the temperature of the blood of a healthy person (his wife), which he measured in the armpit and called 96 degrees.
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And you can get brine down to -5F or so... Pretty close to 0F. From what I remembered, 0F had nothing to do with sea water but was an ice/brine solution.
I have also read this explanation before:
Fahrenheit scale, why is 32 freezing and 212 boiling?
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Daniel Gabriel Fahrenheit (1686-1736) was a German instrument maker who invented the first practical mercury thermometer. Casting about for a suitable scale for his device, he visited the Danish astronomer Ole Romer, who had devised a system of his own.
As it turned out, it was a case of the blind leading the blind.
Romer had decided that the boiling point of water should be 60 degrees. This at least had the strength of numerological tradition behind it (60 minutes in an hour, right?).
But zero was totally arbitrary, the main consideration apparently being that it should be colder than it ever got in Denmark. (Romer didn't like using negative numbers in his weather logbook.)
In addition to the boiling point of water, the landmarks on Romer's scale were the freezing point of water, 7-1/2 degrees, and body temperature, 22-1/2 degrees.
D.G., simple soul that he was, thought this cockeyed system was the soul of elegance. He made one useful change: to get rid of the fractions, he multiplied Romer's degrees by 4, giving him 30 for the freezing point and 90 for body temperature.
Then, for reasons nobody has ever been able to fathom, he multiplied all the numbers by 16/15, making 32 freezing and 96 body temperature. Boiling point for the time being he ignored altogether.
By and by Fahrenheit got ready to present his scale to London's Royal Society, the scientific big leagues of the day.
It dawned on him that it was going to look a little strange having the zero on his scale just sort of hanging off the end, so to speak. So he cooked up the explanation that zero was the temperature of a mix of ice, water, and ammonium chloride.
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So take your pick (I tend towards the second link—with working with too much mercury on the side—older thermometers primarily used mercury for the indicator).
Going under the ocean 5,000' is roughly 150 Atmospheres (~2,200 PSI)--and from the earlier link, this would decrease the temperature water ice would form by about -1C (80 ATMs per -0.55C).
The boiling point of water is much more affected by pressure than the freezing point. And, with super cooling (and heating) you can get weird effects that kind of blur what the "freezing" and "boiling" of water is...
-Bill
