overboard
Synonym discussion
Sense: adverb/1
For this literal sea-travel sense, overboard is the usual word when a person or thing leaves a boat and ends up in the water. Over the side is very close, but it focuses more on the direction and is often used in longer descriptions. Into the water is broader and can describe falling from a bridge, river bank, or pool edge, so it does not automatically suggest a boat. Aboard is the opposite idea: it means on or onto the boat, not in the water. Compare: The sailor fell overboard during the storm. The children stayed aboard while the captain checked the rope.
Etymology
Overboard is a compound of over and board. In older English, board could mean the side or wooden structure of a ship. The word therefore first described going over the ship's side and into the water, and that physical meaning is still the core one today.