The Onedin Line (1971) : 8x3

An ancient cannon Captain Baines believes to have belonged to Henry Morgan and spirited Margarita, daughter of South American president Juarez, join James' latest cargo but on arrival in Margarita's home country James finds that her father is about to be overthrown in a revolution by his brother-in-law. When the rebels attack James and Tom help the Juarezes to escape but the president is shot by a sniper and, as the surviving trio row back to the ship under fire, the cannon comes in very handy. Elizabeth counts the financial cost of putting up ransom money for her brother.