When a ship has no living people on board (or in cabins and holds) and has been unvisited by people for 200 days, it gets a 2% daily chance of being destroyed and turning into a shipwreck.
1 - (1-0.02)^180 = 0.97
200 days + 180 days = 380 days
This means that an abandoned ship has an almost 100% chance to turn into a shipwreck within a year.
I see many ships idling in the harbor, the number much greater than the inhabitants of the town. Will they all become shipwrecks in a year? The game has run for two decades, then there should be more shipwrecks than ships. However, I only see a handful of shipwrecks.