Here's an opinion based not on personal experience, but simply on what I
hope is logical deduction.  The density of a mussel (both shell and internal
anatomy) is greater than that of water.  So, when placed in water, and
filled with water, they necessarily sink.  The only possible way a mussel
could float would be for the shell to be filled with entrapped air.  There
is no way a mussel could fill its shell with air while submerged.  Therefore
the mussels in question must have been stranded high and dry at some point,
allowing the water to drain out of their shells.  Mussels so exposed are
likely to expire rather quickly.  Many species can close tightly, forming a
watertight seal, and retaining water in the shell until they die, the
aductor muscle relaxes,  and the shell gapes slightly, allowing the water to
leak out.  A mussel that was so stranded, and did manage to survive until
the water returned would quickly open its valves and fill with fresh water.
Therefore my guess is that while the mussels described by Mr. Morgan may
have been complete (with the soft parts still inside the shell), it is very
unlikely they were alive.

Paul M.