What is the range of the depth of the oceans??? Is 3-5km reasonable?
Quite reasonable, I would think, but the answer is complex.
There is a wide range in depths. The distribution of elevation
at the surface of the Earth's crust is bimodal, with maxima at
elevations of +400 m on the continents and -4500 meters in
the oceans. I recommend that physics teachers consult books on
such topics before posing problems for their students. In the
case of university teaching that is particularly important
because there may be students in the class who know more about,
for example, the ocean than their physics professor knows. If
that physics professor says, for example, that the ocean depth
averages 10 km over two-thirds of the globe it will convey an
impression to the students that physics does not deal with the
real world.