When something is watertight, it's sealed so securely that no liquid can get in or out. If your goggles aren't watertight, your eyes will sting after your swimming lesson.
The porthole windows in the side of a cruise ship have to be watertight so no water gets in during storms and the ship continues to float, rather than sinking. And when you brave the flume ride at a local amusement park, you might want to seal your wallet in a watertight pouch. Figuratively, something watertight stands up to any possible questions or doubts: "Her alibi is watertight — her grandma swears they were baking cookies all day."