Well, hydro pump has a mediocre 80% accuracy, so water spout may be preferable when at full health. The problem with that set is that, unless you're going to use it as a lead (and in this case you need a timid nauture to outspeed Aero), you'll rarely be able to abuse a full power water spout because of entry hazards and sandstorm. Also, being unable to outspeed positive natured base 130 even with a choice scarf is really disappointing, so despite the loss in power, you might want to use a timid nature and at least 192 Spe EVs, to hit 396 Spe with the scarf attached. This spread also ensure that you'll outspeed max speed adamant Gyarados after a DD, so you can finish it off with hp electric.
To be honest I see really little uses for Blastoise as a revenge killer, especially in OU, as it's largely outclassed by Starmie as a water type, and honestly I'd rather prefer a scarf-Milotic than a scarf-Blastoise, if only for the better Spe and SpA stats.