I had one once. Sold it the day after I first used it. Let's just say that kayak fishing was not for me. Or vice versa.
Be that as it may, lots of my fishing buddies have yaks. Most of them have Hobies with the pedals. They are not for white water, but these guys do everything from secluded and tranquil bass ponds to stripers and blues in the Sound. There are a ton of them that fish the tidal Housy all winter long. If I was to ever consider trying a yak again, (I'm not) I would go the Hobie route.
Plus, you can definitely get away faster in a pedal powered yak.