Undoubtedly no body is to taste water tank in supermarkets with live seafood!
Don't ask me rely on supermarket salespeople. When I ask, they either don't know or can't answer straight. They say "Oh water's fine. Don't worry!" or "There's got to be enough salt in water", and they add "We've been putting shell fish in water for decades, and we still business! Stop worrying!"
From the British Columbia Center for Disease Control (pdf)
"Do not put live shellfish in a closed container or into fresh water (the shellfish will suffocate and die)".
For shellfish like clams and mussels, keeping them in clean, cold, circulated SALT water is also good, because that allows them to stay alive but dormant. Obviously, it doesn't get fresher than live.