When depths are not too great or conditions are not too unsafe, divers can descend into the water to explore the ocean realm firsthand. However, we are creatures evolved to live on land. Unaided by technology, people are about as helpless underwater as, well, a fish on grass.
It is only through relatively recent advances in technology that exploration via diving has been possible. Even with these advancements, severe restrictions remain on the depth and length of time that divers can spend underwater.