Comparing Fresh water and ocean water, each has their differences.
Most notable is animals living in each. The animals that live in the Salt "ocean" water would not survive in fresh water for an extended length of time. Same holds true for fresh water animals. However there are a few species that have adapted and can live in both. Some Salt water animals have also evolved to live in Fresh water.
Humans cannot drink ocean water without dying. The salt in it dehydrates you to the point you die of thirst. In order to drink ocean water you have to desalinate it in one of many ways. Boiling it being one way.
Ocean water also contains every natural element on the planet. If we could find a way to mine the water that is cost effective, we would have a lot more resources.
Ocean water freezes at 26 degrees F and fresh at 32. While only 6 degrees difference in water terms that is huge. It prevents a vast amount of the oceans from freezing over in the winter, of course global warming has helped with that.
As for your comment of only 3% of the worlds oceans is fresh water, it is very true. While there is a lot of surface area of fresh water the oceans are miles deep. The deepest point on Earth is approx 35,000ft below sea level or about 6.75 miles down. Many of the deepest lakes only hit about 2000 to 3000 ft deep. Also when you compare the surface area of the fresh water bodies comparies to the oceans there isn't much.
The great lakes between Canada and the USA contain 20% of all fresh water on the planet. That is enough water to cover the entire USA with 9.5 feet of water. When compared to the oceans that isn't much. So don't be surprised.
Fresh water is lighter than salt water. Therefore, fresh water "floats" on top of salt water. This principle becomes extremely important when considering the drilling of a well in order to tap into the ground water of any island. The weight of the rain water that percolates into the ground...