About 2.5% of Earth's water is freshwater including freshwater lakes, swamps, atmospheric, groundwater, glaciers, ground ice and permafrost. All freshwater originally evaporated off the oceans and saltwater lakes, and the freshwater collected into the streams and rivers into freshwater lakes. A lot of this freshwater flows right back out to the ocean. Some freezes on the tops of mountains forming snow, snow-caps, ice, and glaciers. In order for humans to drink all the water in the oceans, it would first have to collect into glaciers. Otherwise, you couldn't store it. Then your billions of ice age people could drink the meltwater collected by their dams. Over the course of hundreds of millions of years, their frozen waste water would collect back into the ocean. The whole project would involve fine control of Earth's climate to insure all the Earth's ocean water evaporated, then collected as glacier water. An engineering project which might optimistically take a billion or so years. If you brute force the process, and pump the water to the north and south poles, you can do it much faster. What with nanotechnology and such, and a phenomenal, exponential population explosion, the human could drink the ocean in a tiny fraction of that time.