Unlike some deliberately 'rugged' smartphones, the iPhone 5's screen is not designed for operation underwater, or even whilst significantly wet. This means you couldn't, say, take a photograph or check your email underwater. You know, if you had that in mind.
You could always shoot video underwater by starting to record while the screen was dry, then immersing the iPhone. However, the case seems designed more to protect the iPhone from accidental immersion than to give you a full underwater experience.