ocean floating garbage, especially plastics or fish netting, fouls the propeller
a fisherman on board fouls the propeller
a rouge wave overturns the boat
a huge storm overturns the boat
a whale jumps out of the water in an act called breaching so they can see above the water, and lands accidentally on the boat (recently happened to a yacht)
the crew got lost, couldn't read charts
another bigger ship runs into a smaller boat and either doesn't notice or flees the scene
the captain forces passengers overboard
the passengers fall overboard
they might have dropped their compass in the ocean