What you are trying is impossible. Instead, make sure you have that rusty hammer (if not, it is easy to find from where you are).
Then take a deep breath and go all the way to the place where the key is. BUT: one crosspoint before you go down to that place turn right and immediately left where a ladder leads up (maybe it could help using the map). Go up there and use the hammer on the lock. From there, you can take a second dive to get the key.
You find a walkthrough at http://www.canofworms.com/game/indy/index.htm
- but think twice before you use it because you'll be much more satisfied if you solve the game youself.