The only sea in the world that has no land boundary (no coastline) is the:

The Sargasso Sea, in the North Atlantic Ocean, is the only sea in the world without any land boundaries. Instead of coastlines, it is bounded by four ocean currents that form a large circular system called the North Atlantic Gyre.

Location and boundaries:

• It lies in the North Atlantic, roughly east of the Bermuda region and west of the Azores.
• It is bounded on the west by the Gulf Stream, on the north by the North Atlantic Current, on the east by the Canary Current and on the south by the North Atlantic Equatorial Current.
• These currents circulate clockwise around it, so the waters of the Sargasso Sea are relatively calm.

Sargassum seaweed:

• The sea is named after Sargassum, a brown seaweed that floats in large mats on its surface.
• Unlike most seaweeds, Sargassum reproduces and lives entirely while floating, without attaching to the sea floor.
• The seaweed provides food, shelter and breeding grounds for many marine animals, including fish, crabs, shrimp, young sea turtles and seabirds.

Features of the Sargasso Sea:

• Its water is very clear and deep blue, with high salinity and warm temperatures.
• It is relatively poor in nutrients.

Eels:

Both European and American eels travel thousands of kilometres from rivers in Europe and North America to spawn in the Sargasso Sea. Their young then drift back to the coasts on ocean currents.

Environmental concerns:

• Because of the circulating currents, plastic waste accumulates in parts of the Sargasso Sea, forming a "garbage patch".
• The Hamilton Declaration (2014) set up the Sargasso Sea Commission to protect it.

Why not the other options?

• ✗The Dead Sea is a salt lake between Jordan and Israel, surrounded by land.
• ✗The Caspian Sea is the world's largest inland water body, surrounded by land.
• ✗The Sea of Japan is bounded by Japan, Korea and Russia.