Back in October 2016, Aquaman director James Wan was quoted as saying, "It's ultimately a quest story in the spirit of Raiders of the Lost Ark meets Romancing the Stone." One way to interpret that is that Raiders of the Lost Ark implies World War II era, and WWII implies the Golden Age of comics, and that classic material has been referenced heavily — both directly and indirectly — so far in the DCEU. Going back a little farther than that, Wan also said about Batman v Superman and Justice League director Zack Snyder, "He's actually helping me out in some ways with certain things that he's doing with his [films] so that I can have better groundwork with the standalone Aquaman story."
And Snyder and company have been referencing the DC golden age with abandon. Far more so than people realize. There are numerous scenes in Batman v Superman which are lifted directly from Golden Age comics. Shorter moments, not longer scenes, remixed into a cohesive new narrative. So when James Wan tweets from Day 1 of filming Aquaman, with a pic of what appears to be the interior of a submarine, it makes me think of this moment from the very first Aquaman story in More Fun #73 in 1941:
Now… having a submarine in an Aquaman movie for virtually any reason would hardly be the biggest plotline revelation since 1941, but I'll pencil in this theory until we learn more.