Stress (fright, enviromental, sickness, harassment), feeding response (hunger), lack of space, boredom, curiosity, genus (many are more prone to jumping) just to name a few. And yes fish do jump in the Ocean.
The other thing to consider is that when fish living in the ocean jump out of the water, they typically land back in the water. So the issue isn't so much whether or not they jump (some do) it's where they land.