Frankly, if you're not running M-sharp over M-Blastoise, you're not very smart. Sure, they have different niches and sure, they have different offensives, but why use M-Blastoise over literally Starmie. Sure, M-Blastoise is a special wallbreaker with dark and ice coverage, but Starmie has scald (which is much better), and gets psychic STAB. It's true that Starmie is more utility and M-Blastoise has a good ability that boosts its attacks, but with running Starmie you're freeing up your mega slot. This isn't RU where M-Blastoise is top of the viability charts. This is UU, where mons like Zeraora and Rotom are everywhere. While M-Blastoise can run covers for most of its checks, you still have a mega pokemon susceptible to some very common mons. Yeah, it fits the meta currently nicely, but so does M-sharp and M-Aero, both of which I would use over M-Blastoise