@daylily,
If the hole in the plastic watering can is near the handle, in addition to the silicone, you may want to stabilize the area by using some plastic zip ties/cable ties to prevent flexing. You may also want to cut an extra piece of plastic and silicone it on top of the leaking area to add strength. Gotta keep the old tools working because the new stuff in the stores is....um, crap.
