Calculations of the effect of the ship's shadow on in-water irradiance measurement errors were performed with a Monte Carlo radiance transfer algorithm. The algorithm contained the Cox-Munk wave-slope probability function. A simple 3-D model of the rectangular underwater part of a ship was used. The effect was calculated as a function of sea-water absorption, surface roughness (depending on an assumed wind velocity of up to 15 m s^-1) with various wind velocities and directions, length and depth of the ship, distance of instrument from the ship, and bow-to-sun angle.