In fact this all depends to a great extent of the heigth you are looking from and from the cleanness of the water. In a harbour, with muddy and oily water, it is difficult to make out the bottom of a ship when looking at it from the pier. In the open sea, from the top of a high ship it is not so difficult at all to see the bottom of others or underwater objects. I have seen clearly in the mediterranean small fishes and the seafloor in depths exceeding 10 metres. Had a submarine been there, I would have spotted it from the deck of my ship with no problem.
That said, I can't speak about the Pacific but I believe it is mostly a very transparent sea
