Most bank managers fret about bad loans or a run on deposits. Luzia Moraes has to worry about a leak in the hull, bandits and rainstorms that keep clients away for weeks.

Ms. Moraes, a 43-year-old former housewife, is at the helm of a swashbuckling new venture in Brazil—as manager of the first floating bank branch on the Amazon river system. From a riverboat, she peddles banking services in a frontier where people don’t have much money—let alone experience with ATMs, savings accounts or personal loans.