Alright, let me just put this out there now, if it is the lampshade model, and you don't want it, I would like to stake my claim as a fellow mod.
First gen, as in the big CRT ones?
Fish tank?
Boat Anchor?
Flower pot?
The old CRT iMacs should make ideal Maquariums. Just be very careful removing the display. Those things can still pack quite a punch even after they've been turned off for months. If you could find a 15'' LCD display to replace the CRT, with some fiddling, you might be able to get a mini-ITX system running in it.
http://www.dreamerbase.com/forum/index.php?topic=177.0
eMac would be a fun candidate since it used flat glass CRTs instead of the curved screens that the G3s had.
they've got one but i'm not sure if they're junking it.
How water tight would it be for a fish tank?
Not water tight at all. It's more like you put an aquarium in an imac. http://www.imacquarium.com/