Captain Seafoam McDuck, also known as Hugh McDuck, was an anthropomorphic duck.
Captain Seafoam McDuck made a small fortune (in a time when the Clan McDuck had a hard time pulling through) as a merchant sea captain based in Glasgow Port. Unfortunately, in 1753, he signed a contract with crook Swindle McSue without reading the small print; it turned out Swindle would come in possession of all of Seafoam's riches if he failed to meet his end of the contract (delivering a crate of horseradish to Jamaica). Of course, Swindle sabotaged Seafoam's boat, the Golden Goose, which soon sunk.
The impoverished Seafoam, desperate to make his fortune back, turned to piracy in the Caribbean, though he was hardly the most cruel of buccanneers. Becoming quite famous in his profession, Captain Seafoam acquired a decent stash of loot, which his descendant Scrooge McDuck (time-traveling) helped him save from his rival Captain Walrus. In thanks, after burying the treasure, Seafoam willed it to his 21st-century descendant, whoever might be.
Seafoam married at some point and had a son, mild-mannered and eccentric Potcrack, who himself, in turn, fathered Scrooge McDuck's grandfather. He seems to have died in 1776, leaving as only heirlooms the McDuck Watch and his golden teeth. His ghost haunts the McDuck Manor (or, more frequently, the clouds located above it, where the ghosts spend their time playing golf).
Behind the scenes
Seafoam McDuck first appeared (in flashback) in 1953 in The Horseradish Story. Still in flashback, he was featured in 1992's The Last of the Clan McDuck, Chapter One of The Life and Times of Scrooge McDuck, and in its cut prologue. Seafoam eventually appeared in person in Chapter 5, The New Laird of Castle McDuck, in 1993. He appears with a slightly different design in Family Reunion.
Matters of Genealogy
In Johannes A. Grote's Duck Family Tree, due to a mistake in the German translation of The Horseradish Story, two separate characters hold the place of Seafoam McDuck: one the one hand, Seafoam McDuck, under his proper name, is Scrooge's great-great-great-great-great-grandfather. Scrooge's "great-great-grandfather" cited in The Heirloom Watch, who temporarily owned the titular, is listed separately, and depicted through use of a portrait taken from the background of a Barks one-pager, No Noise Is Good Noise (1949) — though there is a definite likeness between he and Seafoam. This version of Scrooge's great-great-grandfather is shown to have married the Chaldean Slave, here curiously implied to be a descendent of Scrooge-Shah and possibly immortal.
Additionally, Grote also presents his Seafoam as the son of an anonymous McDuck who married a daughter of Cornelius Coot, a curious notion which fails to make much chronological sense even if Seafoam is replaced in the proper generation, and derives from Grote's apparent belief that Cornelius Coot lived in the 1600's rather than 1800's.
Since one of Grote's two potential Seafoams (so to speak) is the grandson of the other, the son of the one and father of the other is of unclear placement in the family tree if one tries to square things back into the commonly-accepted genealogy; one may choose to retain his place as Scrooge's great-great-great-grandfather, making him Seafoam's father, or consider that his being the son of the man directly identified by Grote as Seafoam is the more important feature, in which case he would be Scrooge's great-great-great-uncle.
When creating his Duck Family Tree, Don Rosa included Seafoam as Hugh "Seafoam" McDuck, making Seafoam into a mere nickname. This was, however, likely not Barks's original intent: "Seafoam" is used on a legal document in his story, suggesting it was the character's actual first name. This was not the only time Don Rosa made a similar unneeded name change: in his tree, Seafoam's fellow Barks character Whitewater Duck is referred to as Abner "Whitewater" Duck. Interestingly, at any rate, Don Rosa seems to have made this decision pretty late, as in The History of the Clan McDuck, he still referred to the character as Captain Seafoam McDuck.