It is a confused pattern that the waves make in the open sea – a mixture of countless different wave trains, intermingling, overtaking, passing, or sometimes engulfing one another; each group differing from the others in the place and manor of its origin, in its speed, its direction of movement; some doomed never to reach any shore, others destined to roll across half an ocean before they dissolve in thunder on a distant beach.
– Rachel Carson, "The Sea Around Us"
Years ago I took a ride in a helicopter that took off from a ship's deck. I was able (with a tether) to sit with my feet dangling out the door and nothing obstructing my view to the ocean surface. I watched the undulating waves below. At the ocean surface there was little wind that day, so the waves weren't breaking. I thought we were a short distance from the surface, maybe less than 50 m, until a seagull flew beneath us. The bird was a tiny white speck, so we had to be several hundred metres up. The fact that I couldn't tell how high we were is interesting and many other observers have noticed the same thing.
The surface of the open ocean is complex, like the quote above describes. Waves of all sizes occur at the same time. Ocean surface waves are mostly created by the wind; once they are created they can travel a great distance and interact with waves created else-where. The size and speed, thus energy, of a wave is related to the length of sea the wind blows over, called fetch, and the strength of the wind. A large storm lasting several days can create energetic waves. Once these energetic waves move out of the storm area they are called swell. Now the more energetic waves move faster than less-energetic ones resulting in a sorting of waves. Water is great at transmitting waves, meaning these energetic waves can reach the opposite side of an ocean basin where they potentially create great fun for surfers.
Without a point of reference, like a shoreline or a seabird, there is no way to tell what size of motions you are looking at once you reach a certain distance above the water. Obviously, you could tell how far away you were if you were getting sprayed by the water – but by then you are very close. A skydiver over the open ocean wouldn't be able to tell how high they were without an altimeter. If the altimeter failed it could be a potentially dangerous situation.
On the next flight of that helicopter there was an in flight emergency that resulted in a near water landing. Passengers on helicopter that land (or crash) on the water, typically don't make it out. Helicopters usually flip when they hit the water and dealing with getting out can be very disorientating without training. Since there was a passenger on board, the pilots attempted a deck landing (they didn't want to be responsible for a dead passenger). Those of us on the ship were all in our firefighting gear just waiting for a crash – so it was a very serious situation. Although everyone was okay, at that point I decided never to ride in a helicopter just for fun again (even though I am up in them quite often for work).