What happens if the drain cap is forgotten when the boat is in water?

The most accurate answer in the context of the question is that if the drain cap is forgotten when the boat is in water, the boat will fill with water. When the drain cap is not secured, water can freely enter the boat through the drain hole, leading to accumulation of water inside the vessel. This influx can overwhelm the boat's ability to remain buoyant, ultimately compromising its stability and potentially causing it to capsize or sink.

In contrast, while the rubber ball may serve to limit water entry in some systems, if the cap is absent, the primary risk is that water will continue to enter the boat, rather than being effectively prevented by the ball. Furthermore, retaining buoyancy would not occur without proper closure of the drain; therefore, the focus must be on managing water entry to maintain safety at sea.