Apparently, Dewpider/Araquanid doesn't suck as much as it seems. According to a Smogon discussion thread, the Ability Water Bubble doubles the power of all Water moves used by the Pokémon, as well as resisting Fire moves and providing a burn immunity. Maybe still not the greatest thing with on 70 Attack, but a better Adaptability for Water moves like Liquidation is still pretty nice, especially with no Burn to neuter it.
http://www.smogon.com/forums/threads/pokemon-sun-moon-battle-mechanics-research.3586701/page-14
(bottom of the thread and onto the next page)