Water STAB, Rapid Spin, and Dark Pulse are musts. You need your Water STAB as Water moves have excellent neutral coverage, to check Fire-types, and Scald (the best Water move to use on Blastoise) has a handy 30% burn chance, while Dark Pulse is a must to get past spinblockers. Then, on the last slot you can use either Aura Sphere or Ice Beam. Assuming you use max SpA, Aura Sphere OHKOes Greninja, Lucario, Kyurem-B (after SR), and Mega Kangaskhan (75% of the time after SR), 2HKOes Ferrothorn, Tyranitar, and Mega Gyarados (71.2 - 83.9%). On the other hand, Ice Beam OHKOes Dragon-types such as Dragonite (after SR), Salamence, Garchomp, Zygarde, Noivern, while also 2HKOing Roserade, most Goodra, and Togekiss. In general, Aura Shpere is the best option as the Pokemon it covers are more important and threatening.