Trivia: The Anabelle doll from The Conjuring, another film of director James Wan, is visible while Mera is taking Arthur to her submarine ship. Blurry but there.
Trivia: Arthur's father has a book by H.P. Lovecraft on his coffee table. Lovecraft often wrote about things that dwell in the depths of the sea.
Trivia: When Atlantean King Orm tries to coerce the Fishermen Kingdom to join the war in this film, Orm calls the Fishermen leader by the name "King Ricou"; this was a tip-of-the-hat to Ricou Browning, the stunt swimmer who played the Gill-Man in underwater scenes for the 1950s "Creature from the Black Lagoon" trilogy.
Trivia: Although he plays his younger brother, in real life, Patrick Wilson is 6 years older than Jason Momoa.
Trivia: On the coffee table is the H.P. Lovecraft book, "The Dunwich Horror." In it, one of the main characters travels to the town of Arkham, a reoccurring town in Lovecraft's Cthulhu Mythos, which is where DC Comics got the name for Arkham Asylum. In some versions of the Aquaman comics, Black Manta was an orphan at Arkham Asylum.
Trivia: Nicole Kidman agreed to make part of the film because it was filmed in Australia and she is Australian.
Trivia: Although they play mother and son in this film, Nicole Kidman is only twelve years older than Jason Momoa.
Answer: Because what they're looking for is in the ocean. The maps, the ancient texts and markers leading to the trident. It would seem odd to search for Arthur on land then return to the ocean.