I'd always pay for a water view room - not for the view but for the proximity of the pools.
We paid for a waterview room when we went to POR two years ago.
We actually had a pool view rather than a river view and I was initially disappointed the the "view" was really a glimpse of one of the quieter pools provided that you stood at the window (you certainly couldn't see it from anywhere else inside the room), looked across the walkway, over the wall, over the grass, over a hedge and over some more grass. I think there was a fence involved somewhere too!
I vowed at the time never to pay extra for a water view room again.
Last year, we returned to POR, this time without the water view upgrade.
We had a garden view which, again could only be seen if you stood at the window. In this respect, I didn't miss a water "view" at all.
What I did miss though was being able to open the room door, stroll a few yards and be beside the pool. Instead, we had to gather up towels, sun cream, sun glasses, book, wallet, camera, drink etc etc etc and walk half a mile (or so it felt) to the nearest pool.
This year, we've paid for a water view - not for the view but to be close to a pool for those early morning and post-park dips.
As previous posters have said, you actually see very little from your room, wherever it is.
If you have the blinds open, everyone walking past can see you (not good for their health if I've just stepped naked from the shower - lesser things have blinded men!).
If you're laid on the bed (and chosen to risk the blinds open) you can see the sky.
If you're pretty much anywhere in the room other than at the window, you can see the wall of the walkway outside the room.
If you're not fussed about being close to a quiet pool then don't bother with the upgrade. Save your money and buy more Mickey Ice Creams!