It is likely that the g-forces imposed through the medium of the water itself would crush the pilot, whereas air is highly compressible and not as massive.
The cyborgs in Halo (like Mastechief) are supposed to have fluid filled suits and bodies. The fluid solidifies on impact and so protects the cyborg from damage.
This is how Masterchief can jump from an aircraft in the upper atmosphere and survive the fall without a parachute.
It's also why he can't move for some time after landing. (please do not construe that I am saying that Halo is real. It's fiction).
edit on 8/2/2012 by chr0naut because: (no reason given)

