Pretty sure he cast Excalibur into the lake and the Lady of the Lake cared for it before returning it to him later. (making the stone sword Excalibur).
I do recall Sir Bedevere(?) throwing it back in the lake in the end after Arthur is mortally wounded, but I can't recall the bit where Arthur first throws it to the 'watery bint' before that. Having trouble locating my Arthurian books to research that bit...
Well, according to the wikipedia page on Excalibur, depending on the story, sometimes the Sword in the Stone and Excalibur are one and the same, and sometimes they aren't.
