gastrodon with stormdrain is a real threat to rain teams, but the problem is ludicolo. You can learn ice beam to gastrodon, but even if you're boosted with storm drain, you'll have to preshot ludicolo's switch in and you're not even sure to ohko it.
It's basically the same for vaporeon, and quasgire is a cheaper version of gastrodon.
One of the biggest threat to rain teams have always been tyranitar with his sandstream ability which can totally breaks rain team's momentum. Of course it is vulnerable to water, but it's bulky enough to whistand one or two hits, if switched in carefully. Learning him thunder fang is also a good way to surprise rain team users. It can OHK a bad politoed and thus ruin the whole strat of rain team.