
Bradford T. answered 03/23/21
Retired Engineer / Upper level math instructor
The angle of elevation between the man in the water to the man on the ship will be the same as the angle of depression from the man on the ship to the man in the water because opposite angles between parallel lines are equal.
From the perspective of the man in the water,
sin(θ) = opp/hyp = 20/50
θ = sin-1(0.4) = 23.58°
which is also the angle of depression from the man on the bow of the ship to the man in the water.