verb
BAD : I knew that if I fell into the sea, I would sink.
GOOD : I knew that if I fell into the sea, I would drown.
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Sink is used in connection with ships, boats and objects which go down and disappear beneath the surface of water: 'The ship had been holed in the collision and was beginning to sink.'
Drown is used in connection with someone who dies because water stops them from breathing: 'One of the boys had fallen into the river and drowned.'