She's a fine Pokemon to switch into some Swift Swim abusers, but you're asking a lot. You're for one assuming there will be no switching in, just revenge killing. Second, you're assuming Gardevoir will have enough speed to outrun Kabutops, even though they're the exact same base speed. If Gardevoir is running max speed, does she have enough defense to survive that said Rain boosted Aqua Jet (possibly with Life Orb)? Are there any EVs required to make Thunderbolt a OHKO on a 4 HP Kabutops.. or a 172 HP Kabutops?
And she can't counter all Swift Swim abusers. As mentioned, Ludicolo will hang tough against most sets, and Floatzel will always outspeed (threatening with Crunch, or Watefall). She fails against Toxicroak too, or it's risky in many cases. She won't outspeed Jolly variants, possibly being hit by Dark Pulse, or Sucker Punch. If Toxicroak is extremely brave, it can Taunt Will-O-Wisp, but it will most likely prefer hiding under a Substitute.
A little more detail to clarify what the EXACT set should be.. would be great. I think Encore has a place on a set switching into a Swift Swimmer. For instance, Omastar leads the team. You predict either Spikes, or Rain Dance, and you switch in. You'll outpace him either way, and can Encore the setup move, allowing for a setup of your own, or for something else to switch in.