The knee high boots would be of use when launching/landing as your feet and lower legs will not get wet per se, so you would stay warmer, especially after launching. You have to wade out into the shallow water when launching, and if your shoes/feet/lower pants are wet, could be dangerous depending on weather conditions, but uncomfortable to say the least while fishing. could also help keep paddle drip off your lower body as well. Rain pants will work but they don't seal up good at the cuff, so still getting wet when launching or loading up. Light weight waders cover you from the waist down, and supposedly are comfortable (I ? this but never had any on either) and therefore keep your lower body dry and warmer.
I've looked at the knee high muck boots and such, just don't think those are the answer for me, leaning towards waist or chest waders in the breathable version which should be more comfortable and about the same $$ depending on what you get
GO BIG ORANGE !
I meant to behave, but there were just way too many other options available at the time.