Andrew Paul McKenzie Pegman a flotation test can indeed remove poor seeds from a batch of seeds. The floating seeds are, however, poorly filled seeds which is not the same as unviable seeds. Even completely filled (and therefor sinking) seeds can be dead, while some sinking seeds actually are viable.
Ruud Nabben no, floating seeds are definitely unviable and the flotation test has been shown to be as reliable as tetrazolium, with the advantage of being non-destructive :)