Obviously it's very unlikely that Gyara will get a new ability, by the way the threat would be drizzle, not swift swim, but if it ever got drizzle, then it would probably fall under both the offensive - having a free double stab waterfall to dispose of every non grass\water absorb pokemon - and support uber characteristics. This is pure theorymon of course.
Well, drizzle is a whole other story. However, the original speculation was that the Magikarp is forced to evolve, retaining Swift Swim.