A ship can carry more load than aeroplane of same size. why?

density of water is much greater than density of air

Explanation:

when body is in fluid it carries weight which is always equal to upthrust

upthrust is given as

upthrust = density of fluid * strength of gravitational field g * volume of body in fluid

as volume of ship and aeroplane is same and strength of gravitational field or acceleration due to gravity is same hence upthrust depens upon density of fluid.

density of air is about 1.2 kg/m^3 and density of water is 1000kg/m^3 hence ship carries more load