Ok, I can't believe I missed this one. Funnily enough, it had already been mentioned on an old Macrumors post, but I didn't connect the dots.
I think the clearest prescursor to the iMac G3, especially in putting translucency at the forefront of the product, is the Rowenta Surfline Iron.
It was introduced in 1991 in white, but in
1993 they introduced a model with a translucent blue casing, showing you the water level within (and evoking that 'water' feeling that we love). That design, of course, has been copied by almost all irons today.
Here's a
Youtube ad from 1996.
People noticed the resemblance, at the time. One 1998 news article ran: "iMac and Surfline: Separated at birth?"
Rowetta's marketing department made the following statement: "Rowenta is flattered to see that the new iMac design bears a close resemblance to our Surfline iron, which was launched five years ago. We wish Apple all the success with iMac that we have had with our irons."
Of course, Apple itself made no statement on the resemblance.
I think it's case-closed, at least in terms of the translucent colour inspiration of the original iMac. In the 1999 book Icon: Steve Jobs by Jeffrey Young, Apple designers admitted to being influenced by household appliances, kitchenware, and fashion accessories — not just electronics.