squeak-
There is a simple difference between us and the previous generations, without getting involved in speculation about what the metzi’us was in those days.
A sheretz that grows inside the water is mutar to consume, I don’t have the SA offhand but its in the end of the third perek in Chullin and no one disagrees. However this only applies to such a sheretz that never left the water. The whole question only begins in our system of running water, where there are certain points where the water is filtered in ways that make it questionable as to whether it is considered the sheretz never left the water. I never dug very deeply into the shailah and I don’t know all the details – that is probably obvious, but I know that the question has a lot to do with this, and thus your problem about previous generations is resolved. If I didn’t live in New Jersey maybe I’d know more…