Let me start this off by saying that I had a dream last night. In it, my wife and our friends were playing Sea of Thieves; we had a galleon and we encountered another galleon with white sails and no decor crewed by two players only (not unlike the galleon we encountered last night which also had whites sails and no decor). Naturally, it being Sea of Thieves, we started to fire our cannons at each other and when we did ominous music started to play. All of a sudden, a massive whirlpool, maelstrom, appeared between us. We didn't want to get sucked into it and so both ships, whilst still firing at each other, turned and tried to get out. We fired a rigging ball at them and watched as they slowly started their descent into oblivion.
Maelstrom's could be a random event such as the Kraken, megalodon, and cursed ships but only appear in storms.