"Slave" is perhaps not the best word, more like "penal labour". And, yes, transportation to the Caribbean colonies was sometimes used as a punishment for criminals, traitors etc. during the 17th century. While the particular dramatization in Captain Blood is probably not entirely accurate, various forms of labour were used in planter colonies before African slavery eventually became the dominant labour source. Given that the work in Caribbean sugar plantations in particular was extremely demanding and often fatal, forced labour was the most viable option. Once again, keep in mind that this was not really the same as the kind of slavery we associate with sugar plantations or, say, the American South: it was not chattel slavery and children weren't born into it, for example.