It is a difficult situation. Our expectation is that if a rigid object bumps into something that a high impulsive force results. But the water complicates things. In this case we have zero net momentum both before and after the collision. Does a high impulsive force actually result? I am having a hard time wrapping an intuition around the situation.
We have an upward flow of water prior to the collision. So a good question would be what happens to that flow. One answer is that it should stop. But if it stops, there has to be a force making it stop. That force is negative pressure. Negative pressure that should be centered on the impact point and that should negate the impulse from the collision. I think we're going to have to get some cavitation going before we can impart much net force. And even then, it would only be temporary.
[Negative pressure is not unreasonable. If the water is under atmospheric pressure we can have up to 15 psi of negative gauge pressure before we hit zero absolute pressure. Even negative absolute pressure is physically reasonable. Water has surface tension. In the absence of nucleation sites, it will resist forming voids. A quick trip to Google yields http://discovermagazine.com/2003/mar/featscienceof which is less than authoritative, but quite readable.
This hit is more authoritative]