To prevent water from getting under your garage door, you can consider installing a bottom seal or threshold seal. These seals are designed to provide a barrier against rain, snow, dust, insects, and rodents. Threshold seals attach to the floor under the door and create a watertight seal, while bottom seals attach to the bottom edge of the door and provide additional protection. You can choose from different types of seals depending on factors like the shape of your garage door, the climate you live in, and whether or not you have a retainer on your door.