If someone falls 30,000 feet without a parachute; what are the chances of survival if they landed in the water?
Summary: A fall from "any height" may be survivable, depending on what you hit, but with an expectation of severe injuries. A water impact at terminal velocity will in all likelyhood result in immediate death.
The Serbian Flight attendant Vesna Vulović holds the Guiness World Record for surviving a fall without a parchute with an estimated fall height 10,160 metres. However, some doubts have been cast on the accuracy of the record, with claims that the fall was actually only several hundred meters. Vulovic was found partly in the wreckage of the plane, so it can be discussed whether she had a truly "free fall" for the purposes of this question. However, several other persons have been known to survive completely "free" falls from great heights. Although it is hard to verify the exact altitudes, the WW2 aviators Alan Magee, Nicholas Alkemade and Ivan Chisov are reported to have survived falls of about 6700 m, 5500 m and 7000 m, respectively.
The exact altitudes are in any case not necessarily of interest, as it can be assumed that all persons falling from a height of more than about 500 meters will reach terminal velocity, the velocity where air resistance prevents the fall speed from increasing, before hitting the ground. As such, above a certain height, other factors become determining for the survivability of a fall. As has been demonstrated by the listed examples, it is clearly possible to survive a fall at terminal velocity. However, this requires favorable conditions at the site of impact, and will likely result in severe injuries.
Due to the flat surface and the surface tension of water, it is hard to imagine an unprotected impact with water being survivable at terminal velocity. The best chances for survival will likely be in areas with foliage, snow and/or a hilly landscape allowing the kinetic energy to be dissipated through rolling downhill. Of the listed aviators above, Nicholas Alkemade appears to have been the least injured. About him, it is reported that
His fall was broken by pine trees and a soft snow cover on the ground. He was able to move his arms and legs and suffered only a sprained leg
To put it simply... NOT GOOD!!!
US Coast Guard rescue swimmers will not do a free fall deployment from above 200 ft (actually, I think it is less, I cannot remember right now) into water, this is too dangerous.
I personally think you'd be better off trying to find about fifteen feet of soft snow to land in! In reading trying to find additional information with which to answer this question, I stumbled upon someone who said they knew of a pilot in Vietnam who's 'chute didn't open when he ejected. He survived with "only" broken bones and a ruptured spleen. I have included a link that I hope is helpful!https://physics.stackexchange.com/questions/9059/jumping-into-water