[durakot]

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.’"