Probably better in the turtle section of the forum. I'm not aware of any snails that would be harmful to turtles and if there are any I imagine they wouldn't be present in a fish tank where you get watercress from. It would probably be fine, though your turtle may eat it...
I've read that wild snails can carry lung flukes that can sicken and possibly kill turtles that eat them. I believe it is recommended that pet turtles only be fed domestic snails.
Thank you for your advice. It was my first time purchasing watercress for my tortoise and since the snail was for the turtle, I posted here. I am nervous of snails that I have not raised or owned for awhile carrying something. Regelus kept chasing it along the waterline and took its shell off. It created a yellow-white foam that stuck to his mouth so I threw it all away. Cleaned all that foam off his mouth and he seems to be ok. I will probably just end up flushing any other snails I may find on these plants.