Very frequent Uber/Lyft rider here.
The 3 main reasons I take Uber are 1) The cars are clean, 2) the drivers are nice, and 3) it's cheaper than taxis (in most cases). If you can cover 1 and 2, you are gtg in most cases.
Everyone except one driver got 5 stars from me. In my mind (and I assume most others), you start with 5 stars and can only go down from there.
Unless the customer says otherwise, you should always just follow the route Uber gives you (assuming they punched in their destination) or gain their agreement if you'd rather take another route. It'll only piss the rider off if you take an alternate route and it ends up taking longer in distance or time (which means we pay more).
Water is nice, but not expected at all. It doesn't get any bonus points from me...maybe it would others. I think if I requested UberBlack, I'd expect some water, but not UberX. If you are UberX, the cost probably outweighs the value for the driver.
Sometimes I'm very chatty and other times I'm pretty quiet (ex: 4:30 AM rides to the airport). I'd say just let the mood dictate the course here. If the rider isn't talking, they probably aren't interested in chatting. If the driver doesn't seem interested in talking to me, I take the queue and stfu.
Uber doesn't allow tipping, while Lyft does. Lyft also has cheaper rates than Uber, so it offsets a bit.
For Uber (not with Lyft), there seems to be more of an expectation to help with bags. I personally dont care if I load my own bag since I am a capable young guy. I'd recommend offering help to others though. Probably an easy way to get dinged.