A-fib can be so frustrating, can't it? I've had it since I was 17 (I'm now 29) and I go through long periods when it's not a problem and then all of sudden it rears its ugly head. I'm not surprised you have more symptoms at night and after meals. That does it for me, too. I tend to eat my meals earlier and I don't lay on my left side. I also use pillows to prop myself up and this helps as well.
Also, some people have a specific type of a-fib that is triggerd by vagus nerve activity, which is the nerve that is involved in digestion and sleep. Some medicines for a-fib actually stimulate this nerve, making the a-fib worse. So, finding the right medicine is a matter of hit-and-miss until you find something that works for your body.
Also, hydration is key to preventing PAC's (so my doctor tells me), which are the premature atrial contractions that sometimes precede a-fib. Maybe that's why the water works for you? Or, maybe it has something to do with calming down your vagus nerve, if that's the kind of a-fib you have.
Have you talked to your doctor about magnesium supplements? Some people's a-fib symptoms improve by taking extra magnesium. Also, potassium-rich foods might help (orange juice and bananas tend to keep some of my symptoms at bay). Have you talked to your doctor recently about these symptoms and has he or she suggested anything specific to help?
Oh, one last thing - heartburn makes my palpitations worse, too, so I try to stay away from triggers like garlic and lemon. Everyone has different triggers, though, so this is also a matter of experimenting.
Be sure to post again to let everyone know how you're doing. Good luck!
Don't know if I can answer your questions, but I can tell you that my afib/flutter is triggered sometimes by positional changes as well. I've never attributed them to the eating or digestion.. what is true is that all of us with this condition try so hard to figure out what is causing all of this trouble. Sometimes I just don't want to always be thinking about it... but can't seem to help myself! I wonder if you try sleeping with a couple of pillows behind you to see if there are any changes... not sure what else to offer.