First of all, sorry for my English. I know it isn't very good.

Now to my problem. I did all this things with the hammer and the key to get onto the platform beside the ship. Then I dug there and stood on this thing. Then I saw a short video-clip with sand that goes down and it appeared a door. I found this door underwather, but I can't open it. What must I do? Please help me!!!

You need something to pry the underwater door open with. You'll find it on the old airplane. Swim over to it and use your hammer.