My wife and I regularly fish from a Canadian Canoe (an Old Town 'Camper'). We find it quite stable and adequate for esturies and calm close inshore work. Have bought a small anchor and this year invested in an electric outboard that straps on the side - cos your'e always too knackered to paddle back if you have gone too far!
Make is important as many Canadians are not up to use in the sea - make sure you check if it is white water or sea compatible. If you are looking for new or second hand, make sure you find out the make and type, pump it into Google and chances are you will find a review to tell you if it is up to the job.
It's a great bit of kit because you can keep it at home and just pop it onto a pair of roof bars when you want to use it. No launching fees either, unless the owner of where you launch is really really tight!
We bought ours new for £1000, but its a bit of a 'Rolls Royce' job with nice wooden seats and made of a high tech material called roylex. The latter was important as my wife needs to be able to lift her end onto the roof rack and carry it to the water; it only weighs 56lbs. You can get far cheaper new - say £500 and significantly less second hand, but weight may be an issue the cheaper you get so before you buy try lifting it above your head with whoever will fish with you. Discovery and Old Town are both good makes, but you still need to check out the model specs.
Fishing from the canoe is strictly a sitting down afair and I recommend that you invest in cheap telescopic rods as they are far easier to manage in a canoe, either a 7' spinning rod, or we recently found a couple of Penn 'Fire Stick'? boat rods for £16.99, for larger species. Landing anything over a couple of pounds can be fun and a short handle net is also worth considering.
Finally - be prepared for strange looks and sarcastic comments shouted by those who have chosen to persue their on-water fishing activities in more 'traditional' craft!