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. He'd be one of its top-tier fighters in any environment. Plus he's a king (literally a king named Arthur), which could certainly make him a distinctive character with a complex backstory (and let's not forget that even in a movie about superheroes, stories are about characters' personalities, relationships, and emotions, not just their physical abilities).
I think Batman: The Brave and the Bold
's version of Aquaman is one that other versions of the character would be well advised to emulate. He had that superhuman strength and fighting prowess, and he had the pride and confidence of a king, as well as a flamboyant, bombastic personality that made him a fan favorite and a standout in the show's enormous cast. Plus they 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.)