During the impact, when the cylinder hits the surface of the water, the water is set into motion almost instantaneously. This is basically an absolutely plastic collision, as the cylinder instead of bouncing back, continues to move together with the water. This means that the total momentum of the cylinder and the water is conserved. Note that anyone who has understood this, but has used a non-exact (i.e. an approximate) expression for the momentum of water, has received a score of -0.95.
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