When British sea captains transporting convicted felons to Australia were paid for every prisoner who disembarked in Australia, the new incentive system "secured to every [prisoner] who died at least one sincere mourner." Who was the "sincere mourner"?

The sea captain was the sincere mourner.

Explanation:

The fact that the British government paid for every prisoner that made it to Australia meant that every sea captain would be careful not to lose any prisoner on his ship, as the survival of every prisoner was an economical benefit to the captain. The captain will sincerely mourn the loss of any dead prisoner, not for the love of the prisoner, but for the loss of money on his side.