How to get a sawed-off shotgun and nice weapon mods on Liberty Island:
Make your way to the north dock (the area in which you meet Harley Filben). There are a couple ways to get this (just like everything), but this method consumes the least resources. In the middle of the dock will be a pallet with many crates on and around it, one of which is a TNT crate. Carry this TNT to the black and yellow striped area and hop into the water with it. Using your Light Aug, dive down and find the sunken ship just in front of you. There's a door on the ship. Push the TNT so that it's floating on the surface right above this door. Get back on the dock and back away some. Fire a single shot into the TNT crate. This takes care of the ship's door without using any lockpicks. Dive down into the ship and smash the crates inside to retrieve your sawed-off shotgun, an accuracy weapon mod, a reload weapon mod, and 50 skill points for level exploration. Not a bad trade for a TNT crate and a bullet.

Thanks to Revolution readers Smokie and Glubags!