In regards to water I've read that it should be at least 250 feet. The estimated fatality rate is 95-98%. 246 feet will probably suffice, unless you're very unlucky. Unfortunately, like all methods you never can know for sure until you've tried. Some people have lived after jumping from the Golden Gate Bridge, which is about 250 feet, while others have died jumping from only 7 stories.
If there are any boulders in the water it would be best to try and aim for landing on them head first. If you don't die from impact, you more than likely will be unconscious when landing and die from drowning.