Water can be classified into 5 different groups. Within that, there are oceans, seas, lakes, rivers, and canals. There are 5 oceans in the world. You can see on this graphic from towerpaddleboards.com that they are ranked from largest to smallest as follows: The Pacific, Atlantic, Indian, Southern, and the Arctic Ocean. All oceans are saltwater and they make up 2/3 rds of the entire earth's surface. This chart also shows the largest seas in the world. A Sea is a portion of the ocean that is usually located where the land and ocean meet.
The largest sea according to this map is the Philippine Sea at 2,199,000 square miles. The Caribbean Sea with its beautiful teal water is the 6th biggest sea in the world at 1,063,000 square miles. The Gulf of Mexico and the Gulf of Alaska also make the list at 600,000 and 592,000 square feet.