Derp. Sorry, briefly forgot how powerful Skrelp's STABs are with Adaptsbility. Still, if HP Fire has a minimum of 21.7%, then that means HP Psychic isn't even a guranteed 2HKO, so not really worth running just for Croagunk imo.120 SpA Adaptability Skrelp Sludge Bomb vs. 132 HP / 116 SpD Eviolite Croagunk: 5-7 (21.7 - 30.4%) -- 98.1% chance to 4HKO
120 SpA Skrelp Hidden Power Fire vs. 132 HP / 116 SpD Eviolite Dry Skin Croagunk: 5-6 (21.7 - 26%) -- 0.1% chance to 4HKO
There's a difference between a weakness and just taking 1.25x damage. HP Fire has no benefit in hitting Croagunk.