@ FIWI (the person above with the "best" answer. Sorry, but your answer is nonsense. Not to be mean, but I honestly don't know why people with no understanding of the science behind an issue will pretend to be an authority on the subject. Don't you realize you are just spreading misinformation? If you don't know, then please just keep quiet already.
First of all, the volume of a substance is not the only factor increasing its diuretic potential (its ability to dehydrate you). Second, your amounts are way off. An individual with no tolerance to cannabis needs about 5 milligrams of TCH to get high. A habitual user might routinely consume 25 to 20 milligrams easily. If you look at the standardinzed TCH preparations sold legally in states with medical cannabis, you will see that these numbers are accurate. So you don't need "a few milligrams at most."
Worse than this, anyone with an ounce of alcohol in their blood would be stone dead. That is over a 0.6 percent blood alcohol level, and you can't survive more than 0.4 or 0.5. So wrong again.
Finally, the real answer to this question is simple: we don't know. Cannabis certainly does cause dry mouth by inhibiting saliva production by a mechanism unrelated to dehyration (ostensibly by interfering with receptors in the submandibular glands), but it may also cause dehydration as well. The fact is there has just not been any definitive research done on the issue. I tend to think that until it is legalized, there probably will not be any real knowledge developed on the subject. If you are really curious, though, there is a good, scientific way to find out. When you are sober, keep a diary of how much you drink, along with how much you urinate. Drink only water. Then, do the same thing the next day, for the same lenght of time, while smoking pot. Except this time, drink the same amount, and see if you urinate more.
Simple as that.