That is an interesting question. Let’s say a six-foot-tall person is standing in an elevator that has five feet of water in it. Would that make any difference if the elevator fell? Let’s say 100 feet.
Assuming the elevator maintained its structural integrity on impact, and assuming you were completely submerged and holding your breath, the elevated water pressure (due to the high g’s of impact) would probably rupture your eardrums, since you woudn’t have time to equalize the pressure like you would when descending on a SCUBA dive. The air in your lungs/throat/nose/mouth would momentarily compress, resulting in an adiabatic temperature increase. This could be serious: an impact that results in 1 g more than gravity (i.e. you feel 2 g’s instead of the everyday 1 g) will double the hydrostatic pressure of the water, in turn doubling the pressure of the air in your lungs. The air will compress, and the temperature will increase from 98.6F up to 220F. This high temperature would only be sustained for as long as the impact forces. However, if you’re only decelerating the elevator at 1 g, that will be sustained for the same amount of time you were in freefall.
Want to decelerate at 2 excess g’s? The air in your lungs will reach 304F. If you were in freefall for 3 seconds, then decelerating at 2 excess g’s means that this high temperature will be sustained for 1.5 seconds. IANAD, but I’d guess we’re getting into fatal thermal injury of the alveolar tissues, as sometimes happens to people who succumb to fire.
The same adiabatic temperature increase phenomenon happens when you descend during a SCUBA dive, but the increase happens over the space of several minutes (instead of a fraction of a second), so your lung tissues are able to transport that heat away gradually without resulting in a noticeable temperature spike.
Eeep. I’ll avoid elevators full of water from now on!