Samuel "Skiff" Beaman, Jr. is a twelve-year-old boy living in the coastal town of Spinney Cove, Maine. He comes from a long line of swampers, a working-class community that relies on the water for survival. Following his mother's recent death, he takes on the responsibilities of caring for his home and his grieving father. Driven by his mother's advice to think smart and never give up, he focuses entirely on raising money to fix their sunken boat.

Samuel "Skiff" Beaman, Sr. is a former fisherman and boat builder of swamper lineage. Following the loss of his wife, he experiences a severe depressive episode, spending his days drinking beer and watching television in silence. He previously worked as a highly successful harpoon fisherman but currently lacks the motivation to fix his sunken boat or return to the sea.

Tyler Croft is a twelve-year-old boy from a wealthy family in Spinney Cove. He rides an expensive bicycle and has access to a Boston Whaler boat named the Boy Toy. He consistently targets Skiff with classist insults, mocking his swamper heritage and his father's current struggles.

Mary Rose Spinney Beaman is Skiff's recently deceased mother. She came from a different social class than her husband but defended his swamper lineage against her own family's prejudices. Although she is gone, her memory and her core rules for living heavily influence her son's decisions throughout his difficult summer.

Mr. Amos Woodwell is an elderly, mostly retired boat builder who lives next door to the Beaman family. He owns a boat-making shed where he continues to work on small projects. He has a long history with the Beaman family, having taught a young Skiff Sr. how to build boats and offering him a place to stay during hard times.

Jack Croft is a wealthy local fisherman and the owner of the Fin Chaser. He previously employed Skiff Sr. as his primary harpoon fisherman, an arrangement that earned both men substantial money. Their professional and personal relationship dissolved following Mary Rose's illness, leading to a long period of silence between them.

Captain Keelson is a local fisherman and member of the Spinney Cove maritime community. He spends time at Mr. Woodwell's shed fixing equipment and occasionally offers technical assistance. He hesitates to get involved in local personal disputes between town residents.

Devlin Murphy is the owner of the local fuel-and-bait shop. He holds a healthy respect for the local fishermen and extends a professional courtesy to Skiff. He sets up a credit account with a fisherman's discount for the boy, just as he once did for Skiff's father.

Mike Haley is a local mechanic in Spinney Cove. He provides free consultations to the fishing community. He gives Skiff an honest, albeit expensive, estimate for the repairs needed to make the Mary Rose seaworthy again.

Joey Gleeson is a boy in Spinney Cove who spends his summer associating with Tyler Croft. He participates in the group's harassment of Skiff, reinforcing Tyler's classist attitudes toward the local swampers.

Parker Beal is another boy in Tyler Croft's social circle. He rides around Spinney Cove with Tyler and Joey, acting as part of the group that frequently bothers Skiff as he tries to complete his daily work.

Mr. Nagahachi is a specialized fish buyer who purchases high-quality tuna to sell to sushi makers in Tokyo. He arrives at the Spinney Cove docks with a checkbook ready to reward fishermen who manage to catch premium fish. He fondly remembers Skiff Sr. as one of the best harpoon fishermen in the area.