The classified is not only meant for water but it is designed with water in mind. Yes you can overclock any of them yourself and they will all overclock about the same unless you just get a bad card but for $20 you get custom pcb with better caps and better power phase delivery. The classified will run just like the others on air.
FTW also has a custom pcb, the rest of the evga's sc, ssc and regular one just have the reference nvidia board, caps and such.
I agree with what he's saying and heps right about the overclocks, just depends what you deem worth $20-$30. I'm on the 1000Mhz reference card but I have the hybrid unit and backplate and it hits 1500 Mhz with the reference board and the classified, gigabyte G1, MSI wont go much higher than where I'm at...maybe 20-30 Mhz if they're lucky.