Truth. And with Surf/Scald + Slackoff Slowking can be a Major thorn in Entei's side. Check out this beastliness:SubCM is the best set IMHO. Everything else Entei does gets walled to hell and back by bulky waters. I'm also surprised no one has mentioned Slowking, since he's probably the best defensive water type in RU.
120 SpAtk Entei Hidden Power Grass vs 252 HP/0 SpDef Slowking: 22.84% - 26.9%
Additionally, Rhydon does a great job shutting down Entei.
120 SpAtk Entei Hidden Power Grass vs 4 HP/252 SpDef Eviolite Rhydon: 45.45% - 54.55%
And the physical sets rarely carry HP Grass but if you DO decide to fill the last slot with HP Grass:
0 SpAtk Entei Hidden Power Grass vs 4 HP/252 SpDef Eviolite Rhydon: 36.36% - 43.18%
Admittedly, that's a bit more damage than Rhydon wants to take but it is, worst case scenario, a 2HKO. Whereas no Entei ever wants to take a STAB powered Earthquake and Rhydon can take physical hits from an Entai, ALL DAY LONG.