"Water retention" is a blanket term that can be quite misleading. Is it because of estrogen? If so, the AAS isn't to blame, but the estrogen and the way the AAS is being used.
Is the "water retention" simply muscular sarcoplasmic hypertrophy, in which case it's a good thing? If your estrogen looks good, that's almost certainly what's going on. For example, MK-677 makes people gain 15 lbs of "muscle" in 2-3 weeks, that will almost certainly disappear once they cycle off. Many AASs do this, but it's especially pronounced with MK, and it genuinely appears as if you gained extreme amounts of muscle within a tiny window. What's actually happening in the sarcoplasmic cells in skeletal muscle rapidly expanding. Make no mistake, this looks the same as fibrillar hypertrophy, because the sarcoplasm is actual muscle, it's simply portion that responds to certain stimuli at a ridiculous rate.
In fact, they've demonstrated the type of muscular hypertrophy that happens from various stimuli in the literature, and it was found that something like GVT (German Volume Training) induced a rapid expansion of muscular sarcoplasm that other routines couldn't match. So, even if someone is natty, this rapid expansion of sarcoplasmic cells can happen with the right stimuli.
Interestingly, in the same study, it was demonstrated that a higher amount of sarcoplasmic hypertrophy generally meant a lower peak output for the muscle group. One rep maxes dropped, while the higher rep conditioning increased.
In short, water retention = sarcoplasmic hypertrophy alot of the time, and that isn't necessarily a bad thing.
