Well spotted, Chewbacca. Some dwarf mixed it with this topic.
Something is wrong with this topic, I read the question as... (The captain of a big ship was telling this interesting story: "Once I saw two marines standing on the opposite sides of the ship. One was looking to the west and the other one to the east. And they saw each other very well."
How can that be possible?)
But the answer is for a different riddle
I have corrected wording of this puzzle in OP to the following:
A ladder hangs over the side of a ship anchored in a port. The bottom rung touches the water. The distance between rungs is 20 cm and the length of the ladder is 180 cm. The tide is rising at the rate of 15 cm each hour.
When will the water reach the seventh rung from the top?