I recently bought an old plastic canoe that had a couple of cracks in it. In this video, I show how I filled those cracks using pieces of plastic from a milk jug and a cheap plastic welder. This fix isn’t too difficult but does require a plastic welder and some patience. The filled cracks are a bit ugly but hold water well.
The plastic welder I used cost about $15 from Harbor Freight (also available via Amazon here). It isn’t the strongest tool in the world but if you are careful, it should work for you. These plastic canoes and kayaks are made from High Density Polyethylene (HDP) which is a very common plastic. I cut plastic milk jugs into strips to use as a fill plastic but there might be better (thicker, more variety of colors) sources of HDP (ie from plastic buckets).