Roundup is not labeled for aquatic use.
Another product called Rodeo has the same active ingredient (Glyphosate), but is not acutely toxic to fish.

As far as "drowing" cattails, I have read studies that proved that it worked, I have not tried it though. Now would be a good time for me to do it since the lake is low and getting ready to be on the way up with the rainy/hurricane season.