My urologist says 3 litres a day through the works is adequate.
Once you've done kidney stones you worry about things like that.
Hydrated is more a matter of having enough in the system than throughput. You can use diuretics, like beer, to flush through your system but since they promote output you end up dried out despite the input. I pour lots of dilute-to-taste orange squash into myself. I have a p-valve but don't bother to hitch it up unless the dive run time is getting on for two hours or unless there are no facilities to 'go' before you dive.
Since my game plan is to get through most of my 3 Litres during the morning and afternoon to prevent having to get up in the night too often I'm normally well hydrated at dive time.
