The coffee/dehydration theory has been tested pretty thoroughly and it is verifiable fact that it is untrue. Just ask Ann Grandjean for example. The small diuretic effect of caffeine is outweighed by the much larger amount of water you get in these drinks. You could try it yourself if you wanted.

Science works by constantly questioning and challenging ideas. What the alternative health industry calls the "status quo" is not fixed and is subject to change. For a long time the most plausible cause of disease was the miasma theory that it was spread by bad smells until it was shown that were other causes such as bacteria. People (including Batmanghelidj) used to think that peptic ulcers were caused by stress until someone proved that they were caused by bacteria by drinking a beaker-full of helicobacter pylori and becoming very ill. Unfortunately some people continue to stick to old ideas long after they've been disproven, especially if they make money out of them by selling books or miracle "cures". Claiming victimisation from the "scientific establishment" or even threatening to sue for libel when someone questions them is quite common.

Having something to eat or drink is known to temporarily reduce the pain from stomach ulcers. Batmanghelidj decided that this was because of some previously unknown quality of water but I can't find a lot of evidence that he tried anything else to see if it had the same effect. I read through some of his articles at watercure.com last night and some of them show a shockingly bad understanding of how the digestive system works. Essentially he built an entire career out of giving a man with a stomach ulcer a glass of water and repeating his "when I was in prison..." anecdote when questioned about it.

My advice for a beginner? Question everything, especially if it challenges received wisdom. They're either on to something or at best misinformed. Anyone can make claims but being able to prove them is a lot harder.