This fascinating answer says that "The Gemini capsule floated because it had thousands of hollow balls made of extremely thin aluminum, each composed of two half-spheres welded together, the size of ping pong balls, which were embedded between the inner and outer wall, for the exact purpose of flotation". I'd never, ever, heard of such a thing, so I was fascinated.
And sure enough, one of the several (please go take a look) links in that answer show a design of something clearly labeled "ball - flotation". I'll double back to this.
But I'm really skeptical. I guess I'm actually just confused. How would this aid flotation, other than just lightening the mass of the capsule walls? I'm certain they weren't external to the capsule, so they didn't increase displacement.
One of the comments in yet another link in the original answer seems to suggest that they were a reserve of buoyancy in case the capsule [walls] were flooded, which, okay, that makes sense if water gets into the walls, I guess. But the walls of the capsule were obviously airtight, so surely they were watertight too--and I can't imagine that a handful of these things would keep the entire capsule from sinking if, like, the whole thing filled up with water.
Also, look at the requirements in that document: has to resist 1000°F for 30 minutes! What?? Is this a reentry thing? But if the walls get that hot, you've got a huge problem--far, far more so if they get that hot for 30 minutes.
Reading through that discussion thread (the second link) just seems to make things more confusing. Did the things really fly? On Gemini or Mercury? Did they really ever exist? Is it all a scam?
What the heck are these balls??