My hobby is swimming! I like it both in the fresh or salt-water. For me it's both an exercise and recreation.
Can you swim a ocean?
Sure, any kind of salt water like a sea or an ocean will be good enough to swim. It may even be fore survival sometimes or simply as a sport.
Who is the greatest swimmer of all time?
I think Michael Phelps could be considered one. He won the most Olympic Games in the history- 28 medals!