[Answer] What explorer first visited the U.S. river, which is the largest within its state, creating a natural boundary with Mexico?

Step 1 : Introduction to the question "What explorer first visited the U.S. river, which is the largest within its state, creating a natural boundary with Mexico?"

...Hint :Norte

[Answer] Which sea is the only one that does not have a land boundary?

Step 1 : Introduction to the question "Which sea is the only one that does not have a land boundary?"

...If you're cruising through the North Atlantic Ocean, you'll probably run across the Sargasso Sea. It is defined by the North Atlantic Current on the north and the Gulf Stream on the west. The Canary Current defines its boundary in the east and the North Atlantic Equatorial Current defines its southern boundary. Because it's currents that make up its boundaries, the borders of the Sargasso Sea are dynamic. The Sargasso Sea is named after the vast patch of free-floating seaweed, called Sargassum, found there. Source: OceanService.NOAA.gov

[Ans] which sea is the only one that has no land boundary?

Step 1 : Introduction to the question "Which sea is the only one that has no land boundary?"

The sea 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, a clockwise-circulating system of ocean currents termed the North Atlantic Gyre.