Hoping the photo uploaded. I bought standard blue barrels to build floating docks but am unclear on a couple things. The photo shows one of two openings on the top. The other is open to use a bung tool to open. This one, however, has this unmovable plastic lid with a silver seal around it. Is there a standard bung under it? Should I break the cover off to reveal the bung to then screw it out and use some plumbers dope to seal it, as I intend with the exposed one?

Haven seen one like that before but would consider running the opening horizontally and don’t worry . The water level won’t come up that high some folks add water to the barrel to get the ballast where they want it

My gues is that that barrel contained food stuff and that's a tamper proof seal. Underneath I bet you'll find a traditional bung.

Good points on both comments. Thanks

The metal ring is frangible, you cut it with dikes, remove the cap and the bung is std under that.
Believe it or not, Roundup used to come in these-just like that..
Edit: Sorry, didn't read 2old2soon's post..what he said..:-)

I have read here some where that the barrels will leak. You need to use a sealer on the threads of your caps. I would use a marine grade sealer. That is made for plastics. Also place so bungs are out of water as much as possible.

If you have a concern about the barrels leaking over time you can have a insulation company fill them with spray foam, that way it should never be a problem.

LifeCaulk.. most marine dealers have it.