I missed this thread earlier. I like the hanging witches cauldron suggestion.
If you still have the item and can look at it more closely please can you tell us if there are any threads in the opening? Like, could there have been a screw-in plug/lid at one time?
As far as the nautical stuff goes,
the bumpers and floats are usually quite colorful, but the dock bumpers are often black (heck, sometimes they use old tires).
Are there any mold marks on the sphere? I am not talking about the fungus type mold but the marks that are left when something is made in a mold. (Hubby was a moldmaker.) Either the seams or maybe some letters or numbers? Please look inside on the bottom, too.
Just about anything I can think of won't be correct unless there can be a lid (I am thinking of a weight that hangs) or...
we can turn it over and look at it the other way and it can attach my 2 chains. So for now I am guessing a dock bumper of some kind...but it needs a lid or there will be mosquitoes.
One more question please...can you say where this was found? Roadside, industrial area, thrift store, etc. Knowing the location might help.
Sorry for so many questions.