Just looking at things around me, I find a lot of them came from a different continent. My washer was made in Korea; some of the clothes I am wearing were made in Chine, some in India, and some in Indonesia. The chair I am sitting on was made in Taiwan. Some goods travel by air, but air freight is a lot more expensive than shipping in a container by sea.
Much of the oil consumed in the U.S. is imported. Much of the oil produced in the U.S. is exported. Much of this trade is by sea (some is by pipelines to Canada and Mexico).
Just down the road from me is a Walmart store. Over 50% of what is sold there comes by sea to the U.S., mainly from China.
The main evidence of sea trade around here is the Port of Houston. Its importance is shown by the statistics (Economic Impact ):
Direct and indirect jobs: 1.3 million
Direct and indirect income: $19 billion
Taxes $1.5 billion
Just for the Houston area, and for me personally, sea trade is very important.
For the U.S. as a whole, it is important.
For the world as a whole it is very important. Many countries are much more dependent on sea trade than the United States. For example, Australia, which as an island, has to move all heavy imports and exports by sea, and has exports of around $250 billion and imports around $200 billion every year.