Cannabis Culture's Dana Larsen argues that Popeye's "spinach" is really … well, I'll just quote the article:
Since his creation, the pipe-puffing Popeye has become a global phenomenon, with millions of kids heartily munching on spinach in the hopes that it will make them as strong as the legendary sailor-man.
Yet is the spinach which gives Popeye his super-strength really a metaphor for another magical herb? Have children around the world been adoring a hero who is really a heavy consumer of the forbidden weed—marijuana?
The short answer is no, but Larsen has constructed a pretty entertaining case anyway. I especially enjoyed the argument that "the use of phrase, 'I yam what I yam,' can be seen as a reference to Popeye's use of the burning cannabis bush, which creates his higher awareness of the self-reflective nature of the Godhead."
It helps that Bobby London, a veteran of the infamous Air Pirates Funnies collective, took over the strip in the '80s. A decade earlier, the Air Pirates crew had gotten into trouble for producing unauthorized comics in which the Disney characters got laid and got high. Put in charge of the authorized Popeye, London eventually gave into temptation:
The only Popeye strip to ever explicitly refer to the pot/spinach connection was published in the 1980s by illustrator Bobby London. The comic showed Popeye and Wimpy picking up a load of "pure Bolivian spinach."
London did the syndicated Popeye daily strip for King Features from 1986 to 1992, and was known for putting adult, controversial themes into his work….London was eventually fired from Popeye for writing an allegorical satire about the abortion issue.
If you prefer the Popeye of E.C. Segar and the Fleischer Brothers, there's a nice tribute site here.