Heavy coffee drinker here. Little water consumption. Bouts of light-headedness come and go. Heart palpitations for past 20 years, never associated it with coffee consumption. Friend had a heart scare, and was told he needed to drink lots more water. I got the message, and started loading up on water. Light-headedness went away, but palpitations still occurred. (Note: Doctor did blood work and ran tests, but never suggested I stop with the caffeine.) Did some reading and thought the coffee may be involved, so I started to drink decaf only for about 3 cups a day. Brand of regular coffee I used to drink is reported to be higher in caffeine content than most brands. Anyway, now, no more palpitations. I guess some people may be more tolerant to caffeine.
As coffee is 98% water, you are basically drinking water when you are having coffee!
So feel free to add coffee to your daily water consumption. Especially if you can drink it black, which is the best way to consume coffee in my opinion, though it is an acquired taste for many.
I was a "sugar and cream" coffee drinker for decades and then a few years back, I decided to cut out those empty calories and I forced myself to drink my coffee black. It took about 2-3 months to get used to it but now I like the taste of coffee so much that I'll never go back to sugar and cream. I even find myself ordering dark roasts and exotic blends as I can actually appreciate the flavors now.
I keep hearing that coffee (caffeine) is a diuretic but there is no science backing that up whatsoever. So feel free to add coffee to your water consumption totals.