Blissey, Milotic, Suicune and Starmie laugh at that set, Aerodactyl and Jolteon are faster than 252 +1 Modest Blastoise, and Celebi (even with 252+ Modest Ice Beam) can wall you, but provided you're facing a Swampert team without Blissey or Celebi, it should work as much as a Hydro Cannon Blastoise possibly could. Also expect an opponent with DD Salamence or DD Tyranitar to sweep you immediately after you use Hydro Cannon if you manage to KO something with it.
Here's the best I could come up with to work with what you want:
Blastoise @ Salac Berry
Ability: Torrent
EVs: 24 HP / 252 SpA / 232 Spe
Modest Nature
IVs: 0 Atk
- Hydro Cannon
- Toxic / Ice Beam
- Rapid Spin
- Substitute
232 speed at +1 outspeeds Jolly Dugtrio, and also lets Blastoise set up a sub against opposing Waters, with the rest of the EVs dumped in HP. More speed beyond that doesn't help as far as I can tell. Toxic gives Blastoise a more lasting presence by crippling opposing Pokemon, while Ice Beam helps against Flygon, Salamence and Celebi. Ideally you'd want to fire the Hydro Cannon on the turn it would die from passive damage (sand, Toxic/Burn) so the enemy can't abuse the recharge turn.