Flotation fails quietly and then all at once. The warning sign is geometry, not sound.
Every float carries a share of the structure. When one takes on water and stops carrying its share, that load transfers to its neighbours, which then sit deeper. Sitting deeper accelerates their own failure. The process is self-reinforcing, which is why a low corner rarely stays a low corner.
Floats installed together have done the same seasons in the same water. When one has failed, its neighbours are usually further along than they look. Replacing a single unit on a dock with original flotation is the version of this job most likely to be repeated within two years.
Freeboard is how high the deck rides above the water. Noting it at the same point each season is the cheapest possible monitoring, and it will surface a flotation problem long before anything is visibly wrong.
Not durably. Water entered through a breach in the shell, and draining it leaves the breach. The float will take on water again over the following season.