You can also be pretty quiet if you lift your foot straight up behind you. I mean, heal high, toe pointed down as it comes straight up behind you then step out slowly while leaning back a bit for balance and bring it down heal first, toe up. You need a rhythm and a slow deliberate pace to do it but it works pretty good in creeks where there is some rippling noise. In dead calm standing water, best to just keep your feet underwater like already stated.
Also, avoid places where the mud can be real deep or you could get in trouble. I just had a heck of an ordeal last week and almost got stuck in mud/quicksand twice. Even wrote about it here. Check out "A rough hunt that could have gone bad..."