Awesome calcs, thanks for that. The 2HKO on Azumarill is a big one.
I'm still so torn haha. Even more food for thought: Jolly Aqua Tail does more damage than Adamant Waterfall. I'd say it's seriously worth considering on Jolly sets, to make up for the low power of its STAB.
252 Atk Gyarados Aqua Tail vs. 0 HP / 4 Def Mamoswine: 342-404 (94.73 - 111.91%) -- 68.75% chance to OHKO (at 90% accuracy = 62% chance to OHKO, still higher than Adamant Waterfall)
Is the reward worth the risk? What do you think?
I think for a sweeper like Gyarados, accurate moves are better. Gyarados is not a wall breaker, and a single miss can end your sweep, and cost you the match. If Gyarados was Choice Banded, then I would prefer Aqua Tail over Waterfall. But I typically find that most pokemon that like to switch in on gyarados are resistant to water (i.e bulky water/bulky grass types). Most of the time I find myself using Earthquake/Stone Edge/Ice Fang/Bounce depending on your coverage move, which is stronger for Adamant Gyarados.
Here are the list of relevant pokemon that outspeed Adamant Bulky Gyarados in OU but not Jolly :
After +1 DD:
M. Gengar, Greninja, Alakazam, Jolteon, Crobat, Tornadus-T, Scarf Jolly Breloom, Scarf Timid Politoed, +1 max speed Dragonite
No DD:
Jolly Breloom, max speed Adamant Dragonite, Timid Politoed, max speed Adamant Mamoswine
The only one that is a problem here are: Jolteon, M. Gengar, Alakazam (but they typically have sash so even Jolly would have the same issue), Dragonite and Jolly Breloom. So I find that the loss in power for speed isn't really that nesissary.