William Hodgson is kind of an amazing early horror writer, and his 1909 “The Ghost Pirates” makes sea travel scary for the same reason that a haunted house or a dungeon is scary: a ship is an isolated environment. It can be even lonelier than a dungeon, because a ship is frequently months away from the nearest port, instead of just outside of town.
In “The Ghost Pirates”, the isolation is heightened because the ship seems to be drifting into a twilight zone where they can’t count on contacting the natural world:
It was thus that I came to see something altogether unthought of–a full-rigged ship, close-hauled on the port tack, a few hundred yards on our starboard quarter. … Away aft, hanging from the gaff-end, was a string of flags. Evidently, she was signalling to us. All this, I saw in a flash, and I just stood and stared, astonished. I was astonished because I had not seen her earlier. In that light breeze, I knew that she must have been in sight for at least a couple of hours. … How had she come there without my seeing her, before? All at once, as I stood, staring, I heard the wheel behind me, spin rapidly. Instinctively, I jumped to get hold of the spokes; for I did not want the steering gear jammed. Then I turned again to have another look at the other ship; but, to my utter bewilderment, there was no sign of her–nothing but the calm ocean, spreading away to the distant horizon.
The ship is drifting into another plane – possibly the shadowfell. In the shadowfell, there are ghosts. And in a book called “The Ghost Pirates”, those ghosts are possibly pirates.
“My idea is, that this ship is open to be boarded by those things,” I explained. “What they are, of course I don’t know. They look like men–in lots of ways. But–well, the Lord knows what’s in the sea.”
And that’s the advantage of the sea as an adventure location: your ship might be skimming above empty sea beds, sea monsters, or a nest of Chthulhus. The Lord knows what’s in the sea.