Delta P

This guy is incredibly lucky. I hope that council worker was taken to task over it. Delta-P is not to be messed with. ▶ this video

Similar accidents happen in the water. Pressure differentials are a constant threat in commercial diving operations. ▶ this video

Here an actuall video on the topic: ▶ this video . It is the pressure difference that results on a large force on any unlucky diver.

I've helped float a 70ft steel wreck off of the Caribbean Sea floor using second hand airbags that cost no more than a few thousand dollars (EDIT: You'll want to spend a bit more if this isn't just for fun). Physics is awesome, and the only cost is your time and the materials. Also, if you were only interested in the most valuable materials on the vessel that are accessible without entering a confined space, you'd just need a scuba rig (very low barrier to entry). I would definitely not attempt extensive disassembly underwater. Far more dangerous than floating the vessel and breaking it on shore, dry dock, low tide. EDIT: I thought I'd add why its not safe underwater. Help yourself to some Delta P nightmare fuel: ▶ this video (also confined spaces, unstable underwater structures, etc)