I have the Trayak from Tony's Trailers. Last summer I did my first longer tour with it, some 600 kms with the trailer (and kayak) on tow. It worked like a charm.
I have my doubts about normal kayak / canoe cart with bike (I looked into it when considering the Trayak). For most models I don't think the bearings are durable enough for the speed and distances. Also, pneumatic tyres make the ride smoother and are required by the law where I live.
If the cart has good bearings and tyres and you're towing an open-top canoe, you could fit a yoke of sorts between the bow of the canoe and seatpost of your bike and be all set. Swedish canoe manufacturer Linder (
http://www.linder.se) even makes such a yoke. Some legal issues may remain, around here for example a safety wire is required around the seatpost as well. That would be easy enough.
But this method is less practical with a closed top kayak, because you need to come up with a way to reliably attach the yoke to the bow. This may prove to be difficult without permanent modifications to the bow.
--J