Although I've never done the experiment, the boiling temperature below sea level should increase. This is because the atmospheric pressure increases below sea level similar to the effect of its decreasing above sea level. According to the pressure vs. elevation table I found at:
the pressure is about 114 kPa at 1,000 meters below sea level. Then, according to the 'Boiling Point Calculator' at http://www.trimen.pl/witek/calculators/wrzenie.html
the boiling point would be about 103 °C.
The underlying physics idea is that the boiling point of any substance is the temperature at which the liquid phase changes to the gas phase. The phenomenon called boiling is the formation and growth of vapor bubbles within the liquid. The pressure (for a given temperature) sets the density of molecules in the gas bubble. That sets how rapidly molecules leave the gas and re-enter the liquid. The higher the pressure, the more rapidly molecules must leave the liquid if they are to beat the rate at which gas molecules are going back in. The temperature has to go up to raise the rate of molecules leaving the liquid. So at higher pressure, the boiling point is higher.
LeeH and Mike W.
(published on 05/13/09)