I don't suspect it's due to dehydration, because I've become more and more sensitive to it over the years. I used to be very addicted to Mountain Dew when I was in my early 20s, to the point that was mostly all I drank. But nowadays, I can't even have a single can without really regretting it intestinally the next day. But I have general immune/intestinal issues anyways. It specifically seems to be related to the level of carbonation. Although throw in higher acidic content, e.g. MD, and it makes it that much worse.
I'm fine with drinking tea, so as far as the diuretic part, that probably isn't it. Although I couldn't say whether soda's diuretic capability doesn't make a bad situation worse for me.
If I drink a soda, I am naturally drawn to drink more water afterwards, almost like a psychological cleansing because I do feel some guilt each time I break down and drink one. (Generally, I know it's not good for me and it's better all around if I avoid them entirely.) Somehow drinking water makes me feel less guilty, heh. --But it doesn't seem to make me feel any better than next morning.
The reason I feel guilt is because I realize that colon cancer runs in my family and putting things into my digestive tract which are only going to increase the occurrence of inflammation (chronic inflammation increases the risk of cancer development) only makes it that much more likely it may be something I have to contend with in future.