To increase the rate at which your watering can powers-up, repeatedly fill it up at any water source, such as the pond or your farm. Once you have filled it up a desired amount of times, use it once and your experience for it will increase significantly. Even when it is near Mystrile level, each time you fill it will count as one value better then the three to four uses per value it took before. This is easiest if you have a basement, because you can use the watering hole in it to avoid passing time during the day.