That’s caused by the ball’s bouyancy in the water. It’s less dense, you push it down, it wants to pop back up and does so following the path of least resistance–which is any direction but back toward your finger.
A baseball is more dense than the medium into which you’re pushing it.
One flaw with the squeezing analogy is that squeezing involves a temporary storage of energy until it’s enough to overcome friction and all that energy is suddenly converted to lateral motion. Eliminate that friction and the substance moves only as fast as you’re pressing down on it. Try the squeeze experiment with a dollop of whipped cream, or other equally deformable substance.
If you’re having trouble imaging how a human’s able to move any part of their body at 90+ mph, remember that the base of every joint from toe to fingertip is moving from a base that is already in motion. It’s like firing a gun from a car: the bullet moves at a speed equal to it’s exit speed from the barrel, plus the speed that barrel is moving at (because it’s connected to the car).
I didn’t say anything about squeezing. If you are think squeezing, then you need to reread what I said. There is no stored energy here, it is entirely force vectors. I have no trouble imagining a human moving part of his body at 90 mph. There is in fact, more to throwing the ball than normal forces. Forces off the center of gravity of the ball will be split between spin and moving the ball forward. A hand applying force entirely in dimension X can cause the ball to move in dimension Y by applying the force off center.
Oh, we’ve had a few of these threads lately; I lost track of which one I posted that in.
And a watermelon-seed squirt isn’t due to energy storage; it’s due to mechanical advantage from the seed acting as a wedge.