If I'm doing my math and understanding correctly, baby is a DC/away from you 10.5 hours, then sleeps up to 10 hours without nursing? So, on a workday, he is only with you and nursing for 3.5 hours? I'm no expert, but if that's the case, I think him taking that 16 ounces during the day makes sense--he isn't getting any intake during a significant portion of the 24 hours (when he is sleeping) so is getting his entire milk intake in 14 hours rather than 24. My baby was always on the low end of the 1-1.5 ounce/hour while away estimate, but he nursed a LOT at night.
Do you work 5 days/week? Do you nurse on demand on the weekends?
In any case, there is absolutely no need for the water. (Though I do appreciate her intention behind it--to protect your reunion BF -- but water is not needed).