The Pokemon that appear in the Great Marsh are not the same on any given day. There are a list of Pokemon that are always in there, but there is also a list of Pokemon that can be in there, but aren't always. Unfortuntely, Croagunk is on the second list. The best way to see what Pokemon are in the Great Marsh on any given day is to just use the telescopes several times.
Look here for more info on the Great Marsh.