ABS will make it seem like the rotors are not being gripped but the fact is that they are gripping so hard and working to allow the rotors to turn (prevent lock up).
But water can be an issue too...
here you want apply the brakes slightly before you start driving, so that they may dry.
Cool thing about BMWs is that they have a built in sensor to detect rain/water and they pulsate to prevent a slippery rotor/pad contact.
Hope this helps...let me know if you need anymore help.
2007 GTP Sedan
1969 F-100(in pieces)