The boots’ water-tightness is guaranteed via their design, the possible use of a waterproof, breathable membrane, or via the option of a component like waterproof leather.
While waterproofing boots is a tricky matter, you can improve water repellence, i.e. the ability to repel water. For this, sprays known as “waterproofing” can be used. This will help prevent moisture from penetrating the boot material, which will thus absorb less water and thus be effective for longer.
If you have leather boots, you can use waterproof grease to help waterproof your footwear and protect it from stains.