please explain.
Bazz,
Are you an ignorant Academic to need this explained ?

Thanks Individual for your inquiry.
No I am not an academic, just a poor working class sod.
So I am ignorant of these highly scientific matters on how you get containers off a ship where no wharf exists.
It is just if Hasbeen wrote literally or presumed we all knew that
containers are never watertight, after all many yachts have smashed into floating containers.

No Bazz, directly into the bay, & at times sat on a mooring for days.

I never bothered to look closely at them, perhaps they were specially constructed, or had a special sealing system, but that is how they were handled, & I never heard any complaints about water damaged products carried in them.

Thanks Hasbeen, now Individual can claim he knew that all along.
That is amazing, I would have thought it would have been impossible to
have made the doors waterproof, certainly after many hard drops off
trucks etc. I have not had much to do with containers but never
noticed any seals on the one whose door I opened and closed.
Amazing !

Ordinary containers do float Bazz, at least for quite a while. A funny story, way off topic but fun.

My son had to weld one of our old assault ship, either HMAS Manoora or HMAS Kanimbla, I can't remember which, after it hit a floating container off the Queensland coast one night. Glad it was the navy, & not my yacht.

Those ex US navy tank landing ships were immensely strong in the bow. They had to be to drive ashore to unload tanks & vehicles, & survive to back off. We had welded the bow doors fully shut which made them even stronger.

Well this one ran into a container at night. The strong bow was not effected, but hitting it made it submerge. It's buoyancy made it attempt to resurface, which it did, but under the ship. It pierced both the outer hull, & the inner hull, causing flooding of part of void space between hulls, & one compartment of the ship. To the ship it was not an important compartment, but was to the crew. It was the sewerage treatment plant room, which meant no toilets for the 50 some hours it took to steam back to Sydney at reduced speed. He reckoned the stink was horrible.

Unfortunately in a cost cutting exercise the government had leased the Garden Island dry dock to a private company, & was not available for the navy to use to repair the ship. They had to repair the thing themselves, in the water.

They made a coffer dam type box to fit the hull, more or less sealed it to the hull with foam rubber, pumped everything dry, keeping it so with pumping, fumigated the area, & fixed both outer & inner hulls from the inside. He was quite proud of that one.

As it turned out they could have simply pulled into Gladstone or another deep area of smooth water & fixed the thing no problem.

Bazz,
I came across too harsh, apologies, I just had too much experience with these Uni educated experts' lack of common & practical sense. I have had a lot to do with shipping equipment around including containers.
Yes, containers can float for a long time particularly if they have buoyant cargo & if they turned upside down.
If there is no loading terminal such as you'd experience on small islands then containers get delivered by barges which run right against the beach or barge ramps.
I have been out on a rescue where they let the boat & full 44 gallon drums drift away at sea because they're only there to save lives. Another one was a yacht that ran up on the top of a reef at high tide & all they needed to do was wait for the next high tide & float it into deeper water. What did the official in charge decide ? He dragged the boat off at low tide over the coral & turned a perfectly good yacht into a wreck which instantly sank when off the reef.
He was given the authority by some idiot bureaucrat !