Many containers will just sink along with the boat, either because of tie downs or they’re just too dense to float.

Others however can and will float, generally very low in the water which can cause pretty major hazards to navigation. For this reason, many containers will be fitted with salt plugs that will eventually dissolve and allow water to fill the container which will usually be enough to sink it.

However, if the container was sufficiently full of buoyant material, or the salt plug fails, they can float around for a very long time. Sometimes these containers will be salvaged, left to float, or sometimes militarys will use them as target practice with the stated aim of trying to sink them.

As for Lifeboats, generally you want an empty lifeboat to go down with the ship as a bunch of empty lifeboats floating around could draw resources away from the ones with people in them. Plus, most survival craft are pretty securely tied down so that they don’t accidentally release during normal passage or storms.

Most ships are still fitted with self-release life rafts which are fitted with hydrostatic lines that, if the boat was to sink, the raft would be able to break free, inflate, and rocket to the surface if the ship sinks below a certain depth. These are very common on pleasure craft where the boat can sink quickly and may sink before the crew has a change to prepare the raft.