Overall, the ocean is pretty deep; however, its bottom is not flat or uniform, which means water depths in the ocean also vary. The deepest place in the ocean measures 10,935 meters (35,876 feet) and is found in the Pacific Ocean’s Mariana Trench, at a place called Challenger Deep.
Because the ocean is a big place and difficult to study, if you search online to find the depth of the ocean, you may end up with a range of results.
The most-recent estimate of the average ocean depth of 3,682 meters (12,080 feet) was calculated in 2010 by scientists from NOAA and Woods Hole Oceanographic Institution using satellite measurements. These measurements revealed that the seafloor is much bumpier and mountainous than previously known and resulted in an average ocean depth that is less than previously calculated.
While satellite measurements have yielded better estimates of ocean depth than we have had in the past, rather than measuring the seafloor itself, satellites actually look at the sea surface and detect changes over features like a mountain on the seafloor. So the picture of the seafloor scientists get is far from perfect and is not a very good resolution.
High-resolution seafloor mapping, such as that conducted on NOAA Ship Okeanos Explorer, is needed to fine tune the satellite data. Currently, we have only mapped about 20 percent of the Earth’s seafloor in high resolution, meaning that estimates of average ocean depth remain just that: Estimates.
In the Shadow of the Mariana Trench - 2016 Deepwater Exploration of the Marianas
Leg 3: A Geological Tour of the Northern Marianas - 2016 Deepwater Exploration of the Marianas
WHOI Study Calculates Volume and Depth of the World’s Oceans - Woods Hole Oceanographic Institution