Generally speaking, you should test with OPKs later in the day - mid afternoon or early evening. This is because the LH in your system tends to peak then.
The problem with this is just as you mentioned - you're drinking all day and so your pee is more diluted. It is suggested that you 'hold' for a few hours, not drink anything in that time and then test.
If you're drinking that much water, then that is likely why your pm lines are lighter...
Unfortunately this is the annoying thing about OPKs!
hope that helps!
Second that. You're supposed to test with OPK's in the afternoon. I remember hearing the reason why, but now I don't remember, but I know that FMU tests on OPK's can render false positives.
And I am a heavy water drinker too, but on the days when I plan to test, I hold off. Sucks, but I haven't found a work around. I just drink like a fiend after I test!