According to PBS, the loss rate across the Atlantic was about 10-20%, which sounds about right. So we're talking about around 100K deaths, not hundreds of millions.
Remember, the slaves were valuable cargo, and the captain would only be paid for the ones who arrived in acceptable condition, so the crew tried hard to keep them alive and healthy.
damn right... i once read some imbecile say they threw so many dead slaves overboard that to this very day it has changed the pattern of shark feeding grounds so that they still follow along sea lanes the slave ships took looking for dead slaves!!! words fail