Where Do You Find the Dopefish?
In the words of Zag, "In his natural, underwater habitat".
You'll want to load up H2O, and get to this point on the map, the exit by Landing Pad 3.
Next, you'll want to get to about this part of the map. See that rock on the right?
You'll want to go towards it and check out the back side of it.
As soon as the text pops up, you should get the achievement. I would include a screenshot of where this is on the map itself, except this is too far off the top of map to actually be seen on.