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Thanks to everybody who replied to my question, and sorry for the late
summary. Apparently mosty of the time, the "water added" means that the
scallops have sat around in water to make them plumper, but there can be
more serious things done to them. Here's an excerpt from Bobbi's message,
which was the most informative (thanks!):

>On at least a couple of different cooking shows, the chefs warned
>that some scallops are bleached (?!) to look prettier, and may have
>water injected to plump them up. None of them mentioned whether the
>water contained additives. Their concern was that these scallops were
>usually not as fresh, not as tasty, and would not caramelize (brown
>nicely) if seared in a hot pan because all that water would come out
>and just steam them. Another problem one described might be taking a
>piece of firm white fish fillet or other cheaper seafood, punching
>out scallop-sized pieces, and fraudulently calling them scallops
>(these could be identified by being all the same size in diameter).
>
>If this were done with that fake "seafood" they use for so-called
>seafood salads, that could be very bad for celiacs, as the fake
>seafood (also called imitation crab, and called surimi in Japanese
>dishes like sushi) is often held together with a wheat base.
>
>They recommended looking for scallops of slightly *varying sizes*,
>with a little *variation in color* from white to tan, which would
>mean natural scallops. And smelling them, to be sure they smelled
>like nothing but clean sea water, not fishy or strong. One of the
>chefs was Ming Tsai, and the others were Mario Batali and/or David
>Rosengarden (sp?), all on the food channel (TVFN, website:
>www.foodtv.com).