Ha I actually made an iOS & watchOS app based on that side of the equation after thinking "what's the minimum input I can provide for good hydration reminders?"

It gives a reminder to drink water any time it's been three hours since you've peed...

https://itunes.apple.com/us/app/pee-see-drink-more-water/id1...

Not super scientific, but less of a hassle than tracking water input and it tends to adapt well to factor in things like exercise where more water is needed. It also tends to even out over time when drinking alcohol or coffee. Urine color can also vary depending on foods, like how B12 makes it fluorescent yellow, so the time factor can be another useful indicator.