I don't agree that it's hard to make Aquaman useful on land. Surviving in the ocean depths with their crushing pressure, freezing cold, perpetual darkness, and abundant predatory animals would require truly superhuman strength, endurance, and senses. Far from just "guy who talks to fish," Aquaman would have to be one of the most physically powerful and indestructible members of the League, close to Wonder Woman's or Superman's power levels.
That's all true but water is still his gimmick, if you don't use it and turn him into another Superman/Wonder Woman type, why use Aqua
man at all?
You're also right that he's a king, but Diana is a princess so the royal member of the Justice league seat is already taken.
... gave him Mera's ability to form weapons out of water. Maybe a movie Aquaman could have the same sort of thing that Young Justice's Aqualad had in season 1, reservoirs of water on his costume that he could form hydrokinetically into blades, a shield, etc. (Much like waterbenders in the Avatar animated universe often carry waterskins on their belts so they always have water on hand to manipulate.)
That's an idea I like, it gives him distinctive waterbased abilities. You convinced me, he just became the sixth member of Takeru's Justice League. But that's it, no one else is allowed to join, too many characters fighting for screentime is not good, six heroes is already pushing it.