Imagine you’re stranded on a raft in the middle of the ocean. All this water. But no fresh water to drink.
Oceans are salty because runoff transports minerals and salts from the surface. When ocean evaporates from heat, salt remains in the ocean and water rises.
Even though rain water washes minerals and salts into rivers and lakes, they are mostly freshwater.
This is because their minerals get washed away and transported to an outlet at the nearest ocean.
So unlike salty oceans, lakes and rivers constantly have the minerals washed away.