"The ocean covers 71 percent of the Earth's surface and contains 97 percent of the planet's water, yet more than 95 percent of the underwater world remains unexplored."
Many obstacles exist keeping widespread ocean exploration from becoming something not extremely difficult.
One obstacle is the pressure under water.
"The deeper you go under the sea, the greater the pressure of the water pushing down on you. For every 33 feet (10.06 meters) you go down, the pressure increases by 14.5 psi (1 bar)."
To efficiently explore the ocean depths, the tremendous water pressure must be dealt with in order to keep electrical instruments working. Additionally, the electrical instruments would also have to be waterproofed.
The deeper into the ocean you travel, the darker it becomes. A vast majority of the ocean is pitch black and receives no sunlight. In the early 1500s, scientists were already looking into space, but no one could look much farther than a few dozen feet deep into the ocean. Because a large amount of the equipment needed to explore the oceans have been invented fairly recently, deep sea exploration hasn't been established like other fields of science.
The harsh foreign environment of the deep sea and engineering challenges that come with it have made it difficult to explore the worlds oceans.