Hory chit, I just finally got access to the Umbral Isles (Lv.42 section of MSQs) and at the Ship Graveyard with Norther Krill, I just pulled a:
HQ Merlthor Goby (8.9) > HQ Wahoo (58.2) > HQ Megalodon (174.1)
That's a Lv.50** HQ legendary... which finally earned me my first A rank (Deep Sea).