Thus, a diver wholly underwater would describe him/herself as 'under the sea' because the surface is no longer particularly visible and the vast body of water is observable.Ĭuriously, the phrases 'within the sea' or 'inside the sea' sound wrong to me. The same goes for fog (under a blanket of fog), a body of water (under the sea), or anything else generally deemed 'unmeasurable' (under a cloud of fear). That's why you are 'under her authority' and not 'beneath her authority,' because there's no clear something you can use to measure whether or not you're actually beneath anything. However, when you are 'under' something, that something is obscure and poorly defined, like a large volume of something. The person on the shore can easily see the surface of the sea as a point of reference, but cannot see the vast body of water (in three dimensions).

Therefore, from the perspective of someone on the shore watching a ship sink, they would say the ship was 'sinking beneath the sea.' We can safely ignore 'surface of the' because 'beneath' requires a definable reference and the use of the surface is implicit when nothing else is specificed (humans being basically lazy, after all).

The point here is that the reference is something clearly identifiable or definable - like the surface of the sea. You can be 'beneath' a specific, defined something, like a mark on a wall or a limbo rod. However, the difference between 'beneath' and 'under' (the link provided by is very good) means there is a perspective difference. Given the specific example of a body of water, such as a lake or an ocean, the intent of the phrases is no different - they both refer to something being underwater.

To Go Beneath the Water by K.L. Miller