Let me answer the second part of the riddle first.
Yes, there is a sea without shores. This sea is nearly 3/4 as big as the Mediterranean Sea, roughly 2000 statute miles long and 700 statute miles wide. It is between 70 degrees W longitude and 40 degrees W longitude and between 25 degrees N latitude and 35 degrees N latitude. If you have a world map or a globe handy, you will see that those coordinates put you in the middle of the Atlantic Ocean. But just as our country is one nation, it is made up of a group of states. So also the Atlantic is one ocean, but it is made up of several very distinct bodies of water. There are currents flowing in the Atlantic. These currents form a ring around the perimeter of the northern half of the Atlantic. These currents flow around a relatively still zone of the North Atlantic.
Sam is taking the Porpoise from the tip of Florida, where it is docked right now, to the middle of the North Atlantic Ocean to the only sea on earth without any shores, the Sargasso Sea. Bermuda sits in the northeastern corner of this sea. It is also bounded roughly by the West Indies and the Azores.
The Sargasso Sea is a huge pool of quiet water. Part of the reason for this is that the area is in the horse latitudes where the winds are often light or calm. Because this water is so quiet it is very clear. Visibility is roughly 200 feet or more. Seaweed grows in huge thick mats over much of the entire sea. These mats of weed offer good protection from many predators. That makes the mats great places for European and American eel larvae to hatch. Loggerhead sea turtles hide in the mats until maturity. A wide range of microbes also call the Sargasso home.
Not only sea weed but garbage collects in the Sargasso's still waters. A huge amount of non-biodegradable plastic now floats among the weeds (possibly forever).
Sam is going to the Sargasso to allow Buddy and Nancy to study the plant and animal life of this unusual body of water.
To get to the Sargasso Sea, Sam will follow the answer to the first part of this riddle, the river without banks, The Gulf Stream. The Gulf Stream is a very large warm and powerful Atlantic current that starts in the Gulf of Mexico, exists through the Strait of Florida, and then follows the east coast of the US and Newfoundland. The stream actually speeds up as it moves northward along North America. It has a flow of 30 million cubic meters of water per second at the tip of Florida and up to 150 million cubic meters of water near Newfoundland. All the rivers in North America that flow into the Atlantic only contribute 0.6 million cubic meters of water per second. This is not a straight-line flow. The Gulf Stream winds and bends on its journey north. And without banks to hold it in place, the stream's current is free to move about like a slow-motion version of a flag rippling in the wind.
The Gulf Stream is also wide, 62 miles wide on average and from 2,600 to 3,900 feet deep. It travels on average at 5.6 miles per hour. The mighty Mississippi has nothing on the Gulf Stream.
The Gulf Stream current splits at roughly 30 degrees west by 40 degrees north. One leg of the split crosses to northern Europe and is called the North Atlantic drift. The drift warms the coastal regions of Europe and even spawns cyclones in its waters and in the air over those waters. The other leg of the split recirculates past North Africa.
The warm Gulf Stream current affects climate from Florida to Newfoundland to western Europe. But the Gulf Stream itself is affected by the environment. Winds cool the stream through evaporation and direct cooling. The stream waters become denser and saltier as they cool. These colder waters eventually begin to sink like a lava lamp. These colder, sunken waters become part of the North Atlantic Deep Water current which moves south through the Atlantic.
Sam and the crew should have a fascinating time travelling these waters within waters.