Love my Muck boots, and for standing around or wading in the rivers when fishing, they are great. The stated sizing is actually a little off - I normally take a 11.5 to 12 Medium hiking boot, and my Mucks are labeled as 11M. The fit is perfect for hiking. Mine are rated to -60 degrees and are wonderful in mud, snow, on river rocks, etc. That said, they are definitely at a disadvantage when the water starts to get deeper. Splashes up onto your pants rapidly results in wet legs, unless you have really tall gaiters. Creeks with just a trickle have a way of turning into torrents over night in the mountains, when it rains.
I would get a pair of Lacrosse hip boots, either the Trapper or Big Chief, which will fit tighter and provide ankle support.. They have a nice lining (better than the base model canvas) and removable foot pads so you can dry them easier....The insulated Big Chief even has Air-Bob soles for better traction.