Okay, wow... I found it. It's the lid to one of these!!
Thank you all for your input, without it I may not have happened to stumble upon the solution!
It was found 'left behind' in a home that was purchased. Not much context there, I know... There are no openings in the cylindrical compartment, once the lid is screwed on, it makes a pretty good seal.
It could be a float to hold glow sticks for a night dive.
It's obviously something meant to be reusable. Just how is the back of it shaped?
A float for glow sticks is pretty logical, I will look into your line of thought!
The end of the tube/cylinder is just sealed, mostly flat. It does look like it was meant to float or sit/rest atop something.
Did you find it on an ocean beach?
It could be a new version of a glass Japanese net float.
It's plastic, I probably should have included that info initially, but I'll check out the net float angle and see if I come across anything similar.
Is the small circle the lid?
@LiterateHiker It Is indeed.
floating chlorine thing for a pool?
Kind of looks like one of those but there is no way for the pool water to interact with its contents.
Pool party floating doobie storage
@gonzo93304 Honestly, that's probably the best alternative use for this!
Useless? Good conversation starter at least.
Possibly both? I've seen a good many things and I'm at a total loss.
How about a few hints--like how did you find it and where, how big is it, is the bottom of the tube open?
The bottom of the tube is completely closed. I wish I had a few more hints myself!
As far as size, the dome-shaped top section is probably 12" in diameter, the opening to the cylinder is around 3" diameter an the cylinder is maybe 10" deep/long.