What is the difference between a Sea and an Ocean?

Question

Does the Ocean flow to the Sea? What are the 7 Seas?

Answer 1

Ocean is one of the vast,deep,open expanses of salt water like Pacific, Atlantic, Indian,    Southern  and Arctic.

Sea is an expanse of salt water partially bounded by land and shallower than an ocean.

Answer 2

The terms are rather interchangeable in some ways, like if you are on a cruise over an ocean, you are still said to be "at sea". There are now 5 "official" oceans, the Atlantic, the Pacific, the Arctic, the Indian, and the recently defined Southern Ocean.  Sailors of old used to say "I've sailed the 7 seas", likely meaning parts of the Oceans, but there are obviously many more bodies of water bearing the name "Sea" in them.   So the difference would seem to lean toward size, but it is quite arbitrary.