We could turn this question on its head and ask why is there life on land? Our best scientific evidence indicates that life on earth began in the sea, and only after millions of years did organisms start to live on land.
The oldest fossils are about 3.5 billion years old. They are of ancient bacteria-like creatures that lived in shallow seas. In fact for billions of years, all life on earth was single celled animals that lived in the oceans. It was only about 5.5 million years ago that multicellular life appears in the fossil record. Shortly after this (geologically speaking- 500,000-1,000,000 years) plants may have colonized the land. So you can see that there has been life in the seas for over 800 times as long as its been on land.
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