The invention provides maritime work equipment and cross arm structures thereof. The ends of the cross arm structures stretch into vertical columns and are connected with interior structures of the vertical columns. Each cross arm structure comprises a circular pipe, a horizontal supporting plate and a vertical supporting plate, wherein the horizontal supporting plate and the vertical supporting plate are crosswise arranged inside the circular pipe and connected with the inner wall of the circular pipe. The horizontal supporting plates and the vertical supporting plates are arranged in the length direction of the circular pipes in a penetrating mode, penetrate into the vertical columns and are connected with the interior structures of the vertical columns. By reasonably arranging cross arm interior structures, the cross arm structures and vertical column connecting structures, interior rigidity of the cross arm structures is reasonably distributed, and the matching relation is reasonable; the problem that fatigue failure points occur at the tail ends of the supporting plates inside the cross arm structures is avoided; reliability of the cross arm structures is improved; the bending moment stress level of roots of the cross arm structures is reduced; and the fatigue strength of the cross arm root structures is improved.