You may be surprised by the answer, because you cannot see very far at sea level.
You can see the moon, but you cannot see Europe from the East Coast or Japan from
the West Coast. Why, you may ask? The reason you cannot see very far is because the
earth is round. To give you a better perspective, look at the picture below.
Fred cannot see very far because the curve of the earth is blocking his view.
The higher you are, the further you can see. Furthermore, if there is an object in the sea like a ship or oilrig,
you may not be able to see where it meets the water, but you might see the top of it.
If you stand on the beach, your eyes are probably between 5 and 6 feet above the sandy beach (depending on your height).
From there you can see about 2.7 miles across the ocean. Now, let's say you go up on a sand dune next to the beach,
taking your eyeballs 50 feet into the air. Then, you can see about 8.37 miles across the ocean.
Now, let's say that you are staying in a beachfront hotel on the 20th floor, 250 feet above sea level.
Then you can see 18.71 miles across the ocean. If you could move the Eiffel Tower to the beach and look
out over the ocean from the top of it, you would be 1050 feet above ground and you could see about 38.34 miles across the ocean.
Now, let's say you have even more magical powers and you can move Mount Everest to the beach and stand
on top of it. Then you would be 29,000 feet above sea level and you could see 201.49 miles across the ocean.
You get the point, the higher you are, the greater distance you can see.
But the earth's very curvy shape limits you from seeing very far.