To capsize is to overturn, and it usually happens to boats. Don't rock the boat, baby, or you might just capsize.
When a boat tips over in a lake, it capsizes, and so does a boy who is sitting on an inner tube when it flips suddenly. Anything overturning in a body of water can be said to capsize. Sometimes vessels that capsize can be righted, like a canoe, and other times they sink to the bottom of the sea. The origin of capsize is uncertain, though it may be related to the Spanish word capuzar, "sink by the head."