A floating keychain that went under was usually working exactly as built. It was specified as a bare fob and then carried as a loaded assembly.

Cause one: the payload was never calculated

The fob floats. The fob plus a house key, a car fob and an opener does not.

Modern car fobs are heavy, and they are the single most common cause.

Cause two: the material drank

Open-cell foam and unsealed cork take on water across a season of real use.

The fob rides lower each month until its payload falls below the keys it holds.

The quiet third cause

A punctured hollow body fills and sinks, and looks identical to a working float first.

Foam cannot fail that way, which is why foam is often the right answer for pockets.

What to ask before you order

The payload in grams, net of any coil, clip or opener you are adding.

The material, and for hollow bodies the wall thickness and how the seam is made.