Why Liz would jump in the ocean (funny and serious)
1. Because Jack is there and she is going to save him because she loves him
2. Because Will is there and she is going to save him because she is so protective of him
3. Because she lost an important clue that will unlock a bunch of mysteries and it's crucial she dive in to get it
4. Because the ship will explode
5. Because Teague is in reality a bastard that just likes making people miserable and convinces Liz she deserves to jump off a ship
6. To escape the ship being taken over by Sao Feng/Davy Jones/Beckett
7. Because all the pressures of doing nothing on the Black Pearl become too much and she has to go cool off.
8. Because her piece of 8 is inside a clam....like a pearl, and she has to prove her worth by going to get it.
9. Because she's going to pulll a Steve Irwin and go investigate a sea creature
10. Because she'll be marooned once again with Jack and this time SHE has to go get the pistol.