Why does the crossing time have nothing to do with the current speed regardless of the angle between the bow and the river bank?

The motion of a ship in water can be divided into two parts, along the river bank and perpendicular to the river bank. The speed of water flow affects the displacement of the ship along the river bank, while the speed of the ship affects the speed of the ship moving along the river bank. Because the two parts are carried out at the same time, that is, the movement time of the ship in both directions is the same, so the time for the ship to cross the river is only related to the speed of the ship along the vertical river bank, and has nothing to do with the speed of the water.