The Sheehund by Brian Carraher - Action, Adventure - A desperate man uses a WWII mini-submarine to commit a diamond robbery in East Tennessee. 104 pages - pdf, format 8)

Great concept.  Is this author around?

While you are doing things partially right here -- beginning right in the middle of the action -- at the same time, you've really got no action.

Your opening is the blandest heist I have ever read.

Juice up that opening.  They almost get caught?  Equipment malfunctions?  Are there guards with a timetable for patrolling?  Something.  Anything.

And take off the masks.  We should be meeting our characters here -- and seeing them in action, cleverly working their way through obstacles.

This is too good of a premise for such a paint-by-numbers opening scene.

I agree,  the story is bordering on high concept. Start with the logline - what force/time restraint is working against the protagonist/in their way? Identify this and incorporate into rewrite.

Gl with this idea
Tony