88 - If Christopher Plummer wins best supporting actor he would be the oldest victor in an acting category, but he's likely to be beaten by Sam Rockwell for Three Billboards Outside Ebbing Missouri.
Del Toro said studio executives heard his pitch about a fairy tale involving an amphibian god and a mute woman, told in the style of a thriller.
With 13 overall nominations, the endearing, off-beat love story was seen as one of the frontrunners for best picture heading into the night.
Del Toro's friends and countrymen Alfonso Cuaron won for "Gravity" in 2013 and Alejandro Inarritu won back-to-back Oscars for "Birdman" in 2014 and "The Revenant" in 2015.
"I remember a time when the major studios didn't believe a woman or a minority could open a superhero movie - and the reason I remember that time is because it was March of past year", said Kimmel. "We should continue doing that when the world tells us to make them deeper".
Del Toro then thanked Fox Searchlight for listening to a "mad pitch" and believing "the movie would be a sure bet".
The cold war-set fantasy thriller stars Sally Hawkins as a mute cleaning woman who discovers a freakish aquatic-human hybrid in a tank at a secret government lab, and helps it escape.
Shape of Water wins best picture at Oscars 2018
Dunkirk wins its third award, for Best Editing.
Actor Doug Jones has previously collaborated with Del Toro on five films, including Pan's Labyrinth, Hellboy and Crimson Peak.
The La La Land star pulled a "Natalie Portman", who called out the "all-male nominees" when she presented best director at January's Golden Globes.
The other best picture nominees are "The Post", "Darkest Hour", "Call Me By Your Name" and "Phantom Thread".
"All of us need to understand the importance of what is real, what is authentic and what is fact", said presenter Greta Gerwig, the fifth woman ever nominated for best director, in one of several sly jabs at the president's "fake news" refrain.
"We hope "Icarus" is a wake up call - yes about Russian Federation, but also about the importance of telling the truth, now more than ever", director Bryan Fogel said. "Because it's honoring your roots, honoring your country".
British actor Gary Oldman, who disappears entirely into the role of Churchill thanks to some convincing make-up and padding, is expected to bring home his first Oscar for best actor, although the movie is another outsider for best picture.