My problem was that my plastic watering can had hole is the side (I may have accidentally hit it with the weed wacker).
Out of habit, I went directly to duct tape. That plan didn't work, even after trying to melt the tape and plastic together with my torch.
Next step was to look on Amazon for a new watering can. But I couldn't pull the trigger. I was throwing away a lot of plastic and buying more: it didn't sit right. So, here's the solution to my problem, and all it took was some sawing, drilling, and Gorilla glue.
