Fishing spots in deep ocean are effectively un-used content. The average player will never be hunting whale or orcas, and the global pool restrictions on them limit the amount produced anyway.
Why not let us have an advanced casting rod (steel based?) to fish liftable sized fish out of deep ocean spots? Marlin, sailfish, tuna, halibut are a couple examples I can think of. Special mechanics aren't required, but something could be added depending on the dev time available.