To get water sword collect all 9 life upgrades and go to hourglass chamber to fight the dahaka at the end of the game.
To get a video, on the level 1 when you enter the castle, jump on the first ledge, turn right and find a box. Hit the box, and then you will be able to find an action packed movie on the main menu.
To get alternate ending, complete the game on Hard Difficulty. The first video can be unlocked by completing the game on Normal or Easy mode.