First the container is sealed and it will float. At that point, it will be a hazard to surface navigation, and the Coast Guard, the Navy, ..., will try to recov

First the container is sealed and it will float. At that point, it will be a hazard to surface navigation, and the Coast Guard, the Navy, ..., will try to recover them or sink them. Or the shipper will. Once the seals break down, the container will become heavier than water, have negative buoyancy (Archimedes Principle), and sink. As it sinks the pressure will increase and any remaining air will be squeezed out and it will sink faster. Also, there really aren't that many containers lost overboard (1,382/yr on average) and few are not recovered. There are a lot more sea mounts, about 14,500.