Disregarding the above, regular blastoise isn't very good as a tanky spinner in OU. It's USABLE, especially with it's movepool buffs, but mega launcher and 135 SpAtk make mega-blastoise incredibly risky to switch in on, allowing you to either nuke their spin-blocker, or get a free spin, depending on how you predict things.
So, IF you're set on using Blastoise, then I'd suggest mega Blastoise + normal Gengar. If not, Mega-Gengar would probably have better synergy with Starmie, Excadrill, Donphan or Forretress as far as spinners go. Tentacruel could work as well, despite the shared weakness to ground and psychic.
Why 252 Sp attack over defence for megablastosie? If you want a Tank this Would work to, just leans to more of a defensive Rapid spinner.
Despite his impressive bulk, Mega Blastoise lacks ANY form of recovery, meaning his abilities as a tank are limited. However, his bulk does let him survive to fire off powerful MegaLauncher boosted moves, hopefully kill hazard setters / spin blockers, and rapid spin. He isn't so much comparable to Forretress or Tentacruel as a support rapid spinner, and is instead more like Starmie or Excadrill in the offensive spin camp.
On that note, an alternative EV spread of 252 HP / 188 Def or SpDef / 64 SpAtk has been suggested as it lets him nuke key threats specifically, and gives him a bit more bulk. However, 252 / 252 / 4 can break down walls easily for your sweepers as well, and is still plenty bulky.