From A Wiki of Ice and Fire
and the location of Greywater Watch
Greywater Watch is the seat of House Reed. It is roughly located in the swamps of the Neck, southwest of Moat Cailin. It is built upon one of the strange floating islands of the swamps and does not stay in the same place, making it impossible for ravens or enemies to find. According to Meera Reed, there is no maester there, nor any knights or masters-at-arms.
Several armies of the River Kings and later the Freys have attempted to conquer Greywater, but none of them could find it.
References and Notes