I'd be willing to bet they were Viviparus malleatus, or a similar Viviparus
species.  These are eaten in some parts of the world, they are large enough
to be worth the effort (up to 50 mm or so), and they thrive in muddy, weedy,
stagnant water like park ponds.  I have been to similar ponds in the
northeast where you could stand on the shore and see at least a couple of
dozen snails.  If it's a sparkling clean pond, you won't see them.  But if
its a trash-filled, stagnant mudhole, they love it!

Paul M.