I"m not sure if you are aware of this but I think that none of the water view rooms would have a castle view - they would be on the side of the hotel facing the inner courtyard and/or lake from what I understand. I think the rooms that would give you a glimpse of the castle are the standard view rooms that face the parking lot and MK from the North Wing. So wouldn't that technically be asking for a downgraded room?
We were there earlier this month and had standard view booked but I think perhaps we were upgraded to water view - we looked out on the inner courtyard and lake. Across the hall where the elevator was located there was a window and you could see the MK from it. Most of the Wishes fireworks seem to go off to the right of the castle however - not behind the castle so it seems that the closer around the curve of the wing you get to the MK the less you might be able to see the fireworks.
The water view will let you see the electrical water pagent on the lake - that is very cool and the kids loved it. We had a room on the 2nd floor very close to the entrance that led to the main building and a very short walk to the Food and Fun Center - very convenient. The laundry is also on the 2nd floor near there - great if you plan to do laundry.