If you go here
and scroll down to "PHIL HENDRIE," you can find almost all of the Waterfalls. It doesn't list their dates of death, but it'll show which episodes they came from. The siblings from the movie haven't been added yet, though.
ETA: Eh, Hendrie's kind of scrambled in there at the bottom, so here's the list:
Free Waterfall Jr. (2ACV15)
Free Waterfall Sr. (3ACV05)
Old Man Waterfall (3ACV05, 4ACV04)
Frieda Waterfall (4ACV05)
Where 2ACV15 is "The Problem With Popplers," 3ACV05 is "The Birdbot of Ice-Catraz," and 4ACV05 is "A Taste of Freedom."