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.
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.