I don't know about a mod to water plants automatically, but there's a mod called "Perfect Plants and Perfect Gardens" by twojeffs. It removes the need to water plants, so you can place them as close together as you want. It will also save your sims money, as they won't need to hire a gardener and, as a bonus, stops weeds from spreading if they happen to sprout from a puddle. You can find it in
this thread.