Hawk of the Caribbean

In the mid-1500s, a ship containing Spanish prisoners being sent to a Spanish penal colony in the Caribbean sinks at sea, but some of the prisoners manage to survive and make it to shore at a nearby island. They organize themselves under the leadership of Juan Olivares (dynamically played by charismatic singer-actor Johnny Desmond), manage to take over a ship with some cargo that they barter for supplies, and eventually are asked to fight on behalf of the Spanish crown against the English... or is the request sincere?