Buoyancy should do two things in a boat – provide support and stability.
Firstly if the boat gets totally swamped, the fixed buoyancy should be more than sufficient to float you, your gear and the boat on the surface of the water. If the gunwales of the canoe are supported out of the water, then the water can be removed by bailing. But if any part of the gunwale is under the water then, as you bail, water will just flood back in again, so this must be avoided – by fitting sufficient buoyancy.
Ideally a fully swamped boat will have sufficient freeboard that waves do not come in over the gunwales as you bail. To achieve this you need a lot of buoyancy and you should always carry a substantial bailer, attached with a lanyard so that it cannot be lost overboard.
Secondly, the canoe needs to be stable whilst you bail the water out. To achieve this the buoyancy needs to be located in the ends AND at the sides of the canoe. If you just have buoyancy at one end, that end will be supported but the other end will sink below the water surface. But if you have buoyancy only in both ends, the ends will be supported but any water in the canoe is free to move around. If the canoe heels at all, the free water in the canoe can and will run to the lower side and put your side gunwale under and flood you again or capsize you.