>>37428198This. It's the same reason why, in gen 7, you'll see some Pokemon throwing punches when using Earthquake. They use their physical attack animations because lol it's a physical attack.
With Blastoise, I think it's that it has two special attack animations, a weak and a strong one, but only uses its cannons in the weak one. It fires shit like Water Pulse out of them, but Surf, Hydro Pump, and Hydro Cannon all use the "strong" animation.