Shipping containers are falling overboard at a rapid rate

They have a valve that scuttles them after a few hours otherwise they are a big hazard for other boats

Perhaps some new ones do, but generally most containers have nothing of the sort:

"Depending on whether they are full or empty, and on the nature of the cargo inside, containers may float at the surface for several days or weeks prior to sinking. Containers are not generally entirely watertight; while an empty container is likely to sink due to water ingress, a full container will likely float until air trapped in the cargo has escaped.’"

To anchor discussion, prior to the rise of intermodal shipping containers in the '60s & '70s, cargo was shipped as breakbulk cargo. Losses from theft and damage were considerably reduced by switching from breakbulk cargo to shipping containers. Not the same as losing stuff over the side, but still a regular and somewhat predictable expense.

One anecdote I remember from Levinson's book is about a scottish whisky distiller exporting to the US being very excited about being able to ship whisky in a giant stainless steel vessel inside a container instead of shipping individual bottles inside wooden crates (imagine the theft during loading/unloading...).

That said, shipping containers were not adopted because they reduced theft and damage (consequently the cost of insuring cargo), they were adopted because they offered much lower costs to shippers (after enough investment in ships and ports and cranes and trucks and changes to transport regulation to provide the infrastructure to move containers around efficiently without double-handling them or unloading and repacking them for technical/labour/regulatory reasons).

Marc Levinson's book _The Box_ about the history of the shipping container is worth a read -- https://press.princeton.edu/books/paperback/9780691170817/th...