You have to wear long, waterproof pants along with something waterproof on your feet - maybe you already are, you didn't mention. Though full length waterproof pants tend to be...not nice and cool. Then something genuinely waterproof on you feet - like the Lake waterproof shoes, or actually waterproof covers (not neoprene) like the Goretex Shoe covers they sell ($90 - it's meticulous).

If it's to hot to wear full length waterproof pants, your feet are going to get wet. By "to" hot...a lot of times that's like "anything over 40 degrees". You can slow the water doing what you're doing (wearing something water resistant, along with fenders), but the only other thing you can do is wear something on your feet that will keep them warm, like wool socks.

As far as I know, at least. Tried to find something better, but haven't been able to after a year worth of research.