Gyarados Moveset

I would go for Waterfall. I like synergy.

Awesome, I’m wanting to do this. Turn best friends with a few folks tomorrow. This is my main objective, trade bad IV shiny Gyarados. I have two maxed Kyogre, so I’d keep the DT and give it a novelty Outrage. However, if I didn’t have premier water attackers I’d be tempted to double water it

I’d be inclined to keep it for the legacy move, but if you need water attackers then feel free to TM it. You probably wouldn’t even need to power it up much past 30 to hit breakpoints (you could check the calculator).

The stats aren't all that great, not bad, but you can do better.

You should CTM it to Double Dragon moves and then use it as a defender. Then hunt for a better Magikarp to make double water (15 atk) and power up for offensive use,

Gyarados has high base attack, so its attack IV is a smaller proportion to the total value.
Waterfall has high base power, so breakpoints have smaller percentage increases in damage.

With only 11 atk IV, you may have to power up more to hit a specific breakpoint and you might miss a breakpoint in some situations, but it is not going to matter too much.

https://pokemongo.gamepress.gg/breakpoint-calculator#/
Check the breakpoint calculator to compare some raids you might use it against.
For example, Gyarados' Waterfall against Groudon:

No friendship bonus:
15 Atk IV: 13 damage at level 32.5
11 Atk IV: 13 damage at level 34.5

With best friend bonus:
15 Atk IV: 14 damage at level 30.5
11 Atk IV: 14 damage at level 32.5
15 Atk IV: 15 damage at level 40

So in the extreme case, with 15 atk IV and max friendship, Gyarados can hit one extra breakpoint against Groudon that 14 or lower atk IV can not hit. That is a 15/14=1.07 times increase in damage compared to lower atk IV. But those lower atk IVs are still 14/13=1.08 times as much damage as no friendship bonus.

If you have no other water attackers, this Gyarados will likely serve you very well until Kyogre makes another return to raids, whenever that is.