How long do you think it takes to fish a 20 ton container up from the bottom of a canal and load it onto a barge? There are ~10,000 containers.

Barge? Pull it over the edge and worry about it later.

Depending on contents, some might float very well. Or even dissolve away. My dollar store salt-shakers (with salt) came from vietnam.

The top will be the lightest containers, so I’m guessing that’s where your 40 footer of ping pong balls is sitting.

Yes - that's kind of what I had in mind when I first suggested it. When containers fall of ships they often wash ashore on the UK coast. Even a container of motorbikes arrived in Cornwall recently... I'm presuming that only happens because they have some natural bouyancy.

If they don't tear when they hit the water (the metal is pretty thin) they should float shouldn't they? At 77 cubic meters and 30 tonnes their density is low enough, and they're generally watertight aren't they?

Tie heavy duty straps with a small float attached to it before you dump it over the side, then use a tug to pull it away.