OK, it's black and white, probably a 50's film. Giant attack snails would sneak up on people.
Things I am maybe on:
Snails may or may not have been radioactive
There might have been a dam involved.
Try The Monster that Challenged the World.
http://www.badmovies.org/movies/monsterworld/
That might be it.
Could be The Creeping Terror.
"The Monster that Conquered the World" would also be my guess. The creatures have large snail shells. They sneak up on one old guy in a dark scene, and the canal and lock system of the local waterway figures prominently.