For an abortive high-fantasy-conquistadores game I ran, I had the ocean, real Ocean, be 'black water'.
Literally black. Imagine the ocean as literally black, stretching horizon to horizon.
Inhabited by *things*.
That was the justification for why magical Europe wasn't crossing it effortlessly, and why the players would be trapped in the New World/Carribean; It was a horrible, harrowing passage, could only be made in large convoys with sea priest/druids masking your ships to the best of their ability 24/7, and even then you might lose a ship or to.
The encounters I described in their passage to the players was something that had taken a bite out of a ship from below, breaking its back pulverizing the hull integrity. Those who weren't eaten and managed to stay afloat long enough were pulled out. The ship, so disembowled, sank in less than ten minutes.
The second was a field of blindly groping, tree-trunk sized tentacles reaching from the water. Who knows what they were looking for? They'd occasionally grasp a ship but usually let go... Then they grabbed one and began to wrap around it completely. The understandably spooked sailors began shooting and cutting at it; the tentacles reacted violently. Thrashing around, hard, and did some significant damage to every ship in the vicinity before retreating beneath the waves, leaving everyone in the convoy in terror for the next few days that they'd return.
The PCs were informed that this was considered a pretty good passage. Only lost ~4 ships of the 120-ish fleet, none of the sea priests died.
Then of course they were free to putter around in the balmy, reassuringly blue-water faux-Caribbean, plunder and profit to their heart's content, as long as they realized that anything they shipped back would have to make that journey again.mildly relevant and I wanted to share. Shame the game got nowhere.