Aside from a low SpD, the main problem with an AV Qwilfish, is that it has a huge opportunity cost of losing out on moves like Spikes, Taunt, Pain Split, Toxic Spikes, and Thunderwave. Qwilfish also has an underwhelming attack stat and poor coverage choices outside of its STABs, meaning that it is total set up bait for many dangerous attackers in the tier. If you want a bulky AV water type, Slowking is the best option because it has a really good move pool and SpA backing it up preventing it from being set up bait (Scald / Fire Blast / Psyshock / Dragon Tail).