Answer by taking a quote from the following article:

Stephen McDannell Hillenburg (born August 21, 1961) is an American cartoonist, animator, writer, producer, director, and former marine biologist. He is the creator of the Nickelodeon television series SpongeBob SquarePants (1999-present) which he has also directed, produced, and written. It has gone on to become one of the longest-running American television series as well as the highest-rated show ever to air on Nickelodeon. Born in Lawton, Oklahoma and raised in Anaheim, California, Hillenburg became fascinated with the ocean as a child and also developed an interest in art.

After graduating from college, Hillenburg held various jobs in 1984, including as a park service attendant in Utah and an art director in San Francisco, before landing the job he wanted: teaching children. He hoped to work in a national park on the coast, and eventually found a job at the Orange County Marine Institute (now known as the Ocean Institute), an organization in Dana Point, California that is dedicated to educating the public about marine science and maritime history. Hillenburg was a marine-biology teacher there for three years: "We taught tide-pool ecology, nautical history, diversity and adaptation. Working there, I saw how enamored kids are with undersea life, especially with tide-pool creatures." He stayed at the Dana Point Marina and was also a staff artist. Although "[i]t was a great experience" for him, during this period, Hillenburg realized he was more interested in art than his chosen profession.  While working there, he was asked by one of the educational directors if he would be interested in creating an educational comic book about the animal life of tidal pools. He created a comic called The Intertidal Zone, which he used to teach his students. It featured anthropomorphic forms of sea life, many of which would evolve into SpongeBob SquarePants characters--including "Bob the Sponge", the comic's co-host, who resembled an actual sea sponge, as opposed to his later SpongeBob SquarePants character, who resembles a kitchen sponge. He tried to get the comic published, but was turned down by the publishers that he approached.  At one point during his tenure with the Orange County Marine Institute, Hillenburg started going to animation festivals such as the International Tournee of Animation and Spike and Mike's Festival of Animation where, at one of those, films made by California Institute of the Arts (colloquially called CalArts) students were shown. He determined that he wanted to pursue a career in that field. Hillenburg had planned to take a master's degree in art, but instead of "going back to school for painting", he left his job in 1987 to become an animator.  In 1989, Hillenburg enrolled in the Experimental Animation Program at CalArts. About this decision, he said: "Changing careers like that is scary, but the irony is that animation is a pretty healthy career right now and science education is more of a struggle." He studied under Jules Engel, the founding director of the program, whom he considers his "Art Dad" and mentor. Engel accepted him into the program impressed by The Intertidal Zone. Hillenburg said, "[Engel] also was a painter, so I think he saw my paintings and could easily say, 'Oh, this guy could fit in to this program.' I don't have any [prior experience in] animation really." Hillenburg graduated in 1992, earning a Master of Fine Arts in experimental animation.

What did he do after The Intertidal Zone?
field. Hillenburg had planned to take a master's degree in art, but instead of "going back to school for painting", he left his job in 1987 to become an animator.