In 1981, Forrest Gump (Tom Hanks) sits at a bus stop in Savannah, Georgia. As a feather floats down toward him, he picks it up and recalls his childhood in Greenbow, Alabama during the 1950s, being raised by a single mother (Sally Field), and having to wear leg braces. Despite being intellectually challenged, Forrest is admitted to public school. On his first day of school, Forrest meets Jenny Curran, who becomes his best friend and is also a victim of child molestation. With Jenny's encouragement, Forrest runs away from a group of bullies, struggling until his leg braces break off and he is able to run very fast. Years later, while fleeing the same group of bullies, he runs onto a football field during a practice observed by legendary coach Paul "Bear" Bryant, which gets him into the University of Alabama on a football scholarship and eventually meets President John F Kennedy as a member of the NCAA "All-American" team.
After graduation, he enlists in the army, where he excels at drill exercises and befriends fellow recruit Benjamin Buford Blue, nicknamed Bubba (Mykelti Williamson), an aspiring shrimp boat captain who suggests they go into the shrimp business together after the war. They are sent to Vietnam under Lieutenant Dan Taylor (Gary Sinise). Bubba is killed during an ambush which leaves many of their fellow soldiers wounded. Lieutenant Dan sustains major injuries and loses both his legs. Forrest is wounded in the buttocks while saving members of his platoon and is awarded the Medal of Honor, presented to him by President Lyndon B. Johnson at the White House. At an anti-war rally in Washington, Forrest reunites with Jenny, who has joined the Hippie movement after being expelled from college over topless photos of herself and experimenting with drugs. While recovering from his wounds, Forrest discovers an aptitude for ping-pong, eventually playing against the Chinese in ping-pong diplomacy. He runs into Lieutenant Dan, who is now in a wheelchair and has become an embittered drunk and receives Disability pension. Forrest moves in with Dan and they spend the holidays together, with Forrest explaining his and Bubba's plan to go into the shrimping business and his intentions to fulfill Bubba's dream, and meeting President Nixon and causing the Watergate scandal.
After being discharged from the Army, Gump returns to Alabama and makes US$25,000 from ping pong endorsements, which he uses to buy a shrimping boat, fulfilling his promise to Bubba. Lieutenant Dan joins Gump, and although they initially have little success, after Hurricane Carmen they are the only boat in the area left standing, and they begin to pull in huge amounts of shrimp. They use their income to buy an entire fleet of shrimp boats. Lieutenant Dan invests the money in Apple and they are financially secure for the rest of their lives. Forrest returns home when his mother falls terminally ill and stays with her until her death. Forrest donates much of his money to various causes and continues to live in the house where he grew up, taking a job as a grounds keeper. However he is lonely and often thinks of Jenny, who has been living a life of promiscuity and substance abuse. One day, she returns to Alabama and stays with Forrest. He asks her to marry him, but she declines due to her troubled past. However, they make love that night. After she leaves the next day, Forrest decides to go for a run, which turns into a coast-to-coast three-and-a-half year journey, bringing him national attention.
In present-day, Gump reveals that he is waiting at the bus stop because he received a letter from Jenny, who is now living in Savannah and had seen him on TV during his running and invited him to visit. Jenny reveals Forrest to be the father of her child, also named Forrest, and that she is suffering from an unknown virus (presumably HIV/AIDS or Hepatitis C as both were unknown diseases at the time and could be spread through intravenous drug use). Jenny proposes to Forrest, and he accepts. Forrest and Jenny return to Greenbow with Forrest Jr and are finally married; Lt. Dan attends the wedding with his fiancĂŠe Susan and has new prosthetic legs. Jenny eventually dies of her illness and Forrest becomes a devoted father to Forrest Jr. Later, Gump is waiting with his son for the school bus to pick him up for his first day of school. As the bus departs, the feather from the beginning of the film floats off into the air.

Question: Why did Forrest buy a Shrimp boat?
Answer: To fulfill his promise to his friend Bubba.