From my old clamming days, when we made chowder, we would keep the clams cold but no in water. Remember, a clam lives in water and uses a 'foot' which is a piece of muscle to dig its way into the sand. When in water, the natural response from the clam is to 'dig in' so it'll open. If the water it's in is not the same balance (salt/fresh) from where it came, it'll react by staying open as the salinity is not right.
With all that said, we'd set the clams up in a large pot, add water (fresh) and steam the clams open without cleaning. We'd then take the liquid, which had a the clam juice in it now, and strain it or scoop off the top layers to get the juice but not the sand.
Maybe that'll help you in your future endeavors with clams.
And, with shellfish.. if in doubt; throw it out!!