Answer
$2.86m$
Work Step by Step
First, lets calculate when the diver strikes the water. Lets assume the height of water is 0m and diver started 10 m above the water (assume it is $H_{0}$).
So the time passed till the diver strikes the water will be $H_{0}-\dfrac {gt^{2}}{2}=0\Rightarrow 10m-\dfrac {9,8\dfrac {m}{s^{2}}t^{2}}{2}=0\Rightarrow t\approx 1.43s$.
Using this time, we can calculate horizontal distance easily because horizontal velocity of diver doesn't change. So the horizontal distance will be $L=v_{0}t\approx 2.86m$.
