Southern plantation owners in the colonial era could not depend on using indentured servants for labor because

About as much as 60% of the indentured servants would die before being able to complete their contracts (which was often of 5 years). Although at first, some men were still able to claim some land after the years of work, in the 1660s, most of the good land was already claimed by large land owners. After Bacon's Rebellion in 1676, planters decided to stick with slavery instead of using the system that allowed them to abuse workers and become rich (by using a system that abused a person during that person's whole lifespan).

Indentured servants were not much depended in the Southern states during colonial-era because they were part of the contract which allowed them to work for four years after reaching in America and set free with receiving tools and a small plot of land to start a new life as a farmer. As the requirement of the indentured servants increased, it became harder to get them and were expensive. The alternative turned towards slavery,  forced to work in the fields with no contract sign for releasing them.

c it says nothing about connecting those 2 seas.