My container (used for land-based storage) has obviously seen travel, and has nothing beyond a couple corner air vents. None of the other containers I'm familiar without here (they're quite popular for storage) have anything of the sort. That valve is a nice idea, but doesn't really exist in most of the fleet of containers on the ocean.

They are, indeed, a hazard for other boats, though they generally won't "lurk below the surface." Either they're floating from buoyancy of what's in them, or they sink. The increasing water pressure as you descend makes "floating below the surface" a particularly unstable place.