ＫＵＲＯＳＨＩＯのロボットは長さが５ｍぐらいで、深い海の中を時速８ｋｍ以上で動くことができます。
The KUROSHIO robot is around 5m long and can move through the deep sea at speeds in excess of 8km per hour.
I was wondering if 中 was necessary in this sentence. If we omit it, would it be interpreted as the robot moving on the surface of the sea? I would have assumed that the addition of 深い would make the intention clear without needing 中.
It is understandable without 中, but then it could also interpreted as on the sea. So it is better to explicitly say 深い海の中.
And especially since it is the sea is deep (深い), maybe the author wanted to express that the robot can literally move pretty fast through it.
It does not sound natural without the 中. If you wanted to say that the robot moves on the surface of the ocean then you might use 上 there instead.
