You are asked to find the initial velocity.
Because the drain is horizontal, the initial vertical velocity must be zero (which means the initial velocity is purely horizontal).
Given that
, integrate to get
(where
is the initial vertical velocity - zero)
Then you can integrate again to find the time at which the water hits the ocean - which is the time that the maximum range of the projectile (given to you as 10m) is reached.
Then one substitution into the equation for horizontal displacement will give you the initial velocity.