need help after you swing over from crain to island,dig up switch and step on it.Don't know what to do next? can you help???

You have opened a new door under water close to where you came in.I thought it showed when you stepped on stone.It has something that looks like two columns one on each side. It still is locked and you have to take the hammer ( have you found it?) and hit propeller on airplane underwater,the propeller will open door.