Both characters are monarchs of the Seven Seas, but Aquaman and Namor also share the distinction of taking part in the first Marvel/DC crossover, even if it wasn’t exactly “official.”
Created by Bill Everett, Namor the Sub-Mariner made his debut in 1939’s Marvel Comics #1, one of the very first superheroes in the Marvel Universe. Conversely, Aquaman debuted in 1941 in More Fun Comics #73, created by Mort Weisinger and Paul Norris. The creation of Aquaman was clearly influenced by the success of the Sub-Mariner, and the two characters have gone on to become the pre-eminent aquatic-themed superheroes of Marvel and DC, respectively.
Aquaman/Namor Was the First Unofficial DC/Marvel Crossover
Given that history, it’s only appropriate that the two characters would be Marvel and DC’s first crossover, although the connection was tenuous at best and completely unofficial. The crossover began life when writer Steve Skeates was hired to write Aquaman in the early seventies. Despite his best efforts, Aquaman wound up being cancelled with its 56th issue in 1971. Skeates was forced to end his story abruptly, but was able to continue it in a rather roundabout way three years later. According to CBR, at a 2012 panel at Comic Con International, Skeates related how the crossover came to be: “Since Aquaman had been cancelled abruptly on a cliffhanger, Roy Thomas let (Skeates) wrap up the story in a fill-in issue of Sub-Mariner."
Written by Skeates and with art by Jim Aparo and Dick Giordano, Aquaman #56 begins with the city of Detroit experiencing problems when the sun seemingly never sets, causing algae to grow at an exponential rate and nearly engulfing the whole city. It’s eventually revealed that it’s all the machinations of the ex-cop vigilante calling himself the Crusader, who has developed a satellite with large mirrors that reflects the sun’s light, thus preventing night from falling and keeping crime to a minimum. The story ends in a rather dramatic moment, as the story ends with Aquaman storming the Crusader’s compound and pushing a button which causes the satellite to blow up in the very last panel. It could have ended there, but Skeates picks up where the story left off in Marvel's Sub-Mariner #72, featuring art by Dick Ayers and Vince Colletta. That story reveals an alien life-form attaching itself to a satellite in space just as an unnamed figure wearing green gloves pushes a button to make it explode, causing the alien to transform into a “slime thing” that falls to Earth and battles a brooding Namor. Oddly enough, Sub-Mariner #72 wound up being the final issue of that series, as well.
It’s a strange story made even stranger in how it hand-waves away the character clearly meant to be Aquaman, with the caption in the following panel telling the reader: “Who that man is… and why he wished to destroy the satellite… that need not concern us!” It’s a blatant attempt to keep things as vague as possible so as not to engage in any copyright infringement, but still can’t help but feel underwhelming in the end. Given all their similarities, Namor and Aquaman would be the perfect crossover. Fortunately for fans, Aquaman and Namor would officially meet decades later in the Marvel vs. DC crossover event, where Aquaman soundly defeated Namor by cheating. The two would meet again in the classic JLA/Avengers series, the last time Marvel and DC came together to produce an intercompany team-up.
As far as Marvel/DC crossovers go, the initial pairing of Aquaman and Namor wasn’t the most exciting team-up, yet the unofficial crossover remains historically significant.
Source: CBR