Wasn't the Rock being considered for this role? I think he would have made an excellent Sub-Mariner.
That was the rumor, although nothing was ever officially announced about him being attached to it. He's bound to end up in a DC or Marvel comic book movie eventually. He was attached to star in Lobo
and tentatively attached to be in a Shazam movie (either as Captain Marvel or Black Adam - it hadn't been settled which), but those projects got mired in development hell. It wouldn't surprise me if he ended up playing Green Lantern in Justice League