Originally Posted by Psy-Fi

It's not a submarine, it's a self-propelled semi-submersible vessel. They can't fully submerge. If it went under water, the crew on one would have to abandon ship or they'd drown. No way the Coast Guard would fire on one and risk sinking it and losing that big $ drug cargo.

The smugglers usually scuttle the vessel when they see the Coast Guard coming. They must've been caught off-guard.