[Maltrease]

Here's another idea for clams. What if we treated them similar to the "seeing" orb in LOTR.

You can have a cheap water thaumaturgy spell ("Seeing Pool"... or something) that gives you an "Astral Window" into every provence that contains a clam. Perhaps it could Last for one turn for each clam found in the provence.

You could still make a strategy out of building clams but you wouldn't be able to do it in secret. I'm not sure how many distance attack spells will effect underwater provences, but this would certainly make the land nations think twice before forging lots of clams.