The ship is sailing in the water. Is this a question for you to calculate the speed? For example, the speed of a ship in still water is V, and the velocity of water is v0. Then, if the ship is stable, the total speed is v+v0 and the current is v-v0.

More complicated, for example, two ships are in the water, so regardless of the speed of water, the relative speed and displacement between the two ships are only related to their still water speed.