"Are you saying that it takes more force to lift a measure of mass in the bottom of a pool than it does near the top of the pool? Are you saying tat a rock is less buoyant in shallow water than it is in deep water. "
"SO YOU ARE SAYING THAT WE HAVE BEEN SUCKED IN BY ALL THAT CRAP THAT ARCHIMEDES OF SYRACUSE AND SIR ISSAC NEWTON LAYED ON US IN 212 BC AND 1667 AD RESPECTIVELY. "
The ball is not completely surrounded by water. The forces on the ball include gravity, the weight of the water column (water pressure), and atmospheric pressure. The portion of the ball that is sitting on the seat is exposed to atmospheric pressure with the weight of the water column behind it. Increasing the water column increases the pressure difference and thus the force on the ball. In fact, if you were to increase the pressure so that the force on the ball from water pressure was greater than the force of gravity you could turn the drinking line upside down and the ball would stay on its seat.
If you were to put a ball larger than the neck of a bottle inside a bottle and squeeze the bottle or otherwise pressurize the bottle what would happen? The pressure behind the ball would seat it against the neck of the bottle. Increasing pressure would increase the pressure of the ball against the neck of the bottle. If the ball were compressible you could reach a pressure where you could drive the ball through the neck of bottle, uncorking it.
You are correct that the ball and the seat in a drinking line are incompressible. With perfectly clean surfaces increasing the water pressure and thus the seating pressure of the ball does little to create a better seal. If there is biofilm on the surfaces though (pretty common inside a watering line) then increased pressures may displace the biofilm or other matter from the sealing surfaces ensuring a better seal. And, as I said earlier, increased seating pressures also keep the balls from being rattled loose from the birds pecking at or bumping into a watering line.
For others reading this, that's not to say you should should turn up the pressure to keep the nipples from leaking, you would actually create more of a mess (go back to my post on the basics). We are just picking nits on physics here...