Well Hydration is typically the most useful ability for Swanna. However, since you can not get Hydration I would stick with Big Pecks. Big Pecks is at least mildly useful whereas Keen Eye is almost completely pointless.
Here is a Big Pecks Swanna set that saw mild usage back in the days of BW VGC. Probably not very viable any more but likely the most useful set you will find for a Big Pecks Swanna.
Swanna @ Focus Sash/Lum Berry
Ability: Big Pecks
EVs: 252 SpA / 4 SpD / 252 Spe
- Ice Beam
- Air Slash
Focus Sash ensures you are able to set up Tailwind for support for the whole team. The rest is pretty self-explanatory.