Well since you can't give Blastoise Dragon Pulse or Aura Sphere because it only knows these moves by breeding, and because you have Ice Beam I would give it Dark Pulse.
Blastoise can learn Dark Pulse by TM97 in Pokémon X and Y. Dark Pulse is a really good move for Blastoise, especially Mega Blastoise as it gets boosted by Mega Launcher. It also has a 20% chance to flinch and is super effective against Ghost and Psychic types. With that being said if you were to send in Blastoise with an entry hazard on your side of the field, some players may predict you to go for Rapid Spin to clear away the entry hazards. So they could decide to counter you by switching into a Ghost type, as Rapid Spin doesn't work against Ghost types. You can then surprise them by using a super effective Dark Pulse against them.
It will also come in handy against the following Elite 4 Members and Diantha's Pokémon:
Hope I helped :)