See diagram. P is the plane. C and D are ships.

A = angle of depression made by ship C at P = 60 deg

B = angle of depression made by ship D at plane P = 30 deg

CD = ?

angle A = angle PEC as they are alternate angles between parallel lines and PC is the transversal line.

angle B = angle PDE , alternate angles between parallel lines with PD being the transversal line.

in triangle PCE, Tan A = 900/EC => EC = 900/ tan 60°

in triangle PDE , Tan B = 900/ED => ED = 900 / tan 30°

CD = distance between ships = ED - EC = 1039.23 meters