How do the Knight's govern their islands?
Like, there are probably Knights of Justice serving as governors on each island, but how far in the local government are Knights present. Do Knights patrol the streets? Are the plantations owned and operated at the behest of the Knights, or are there private citizens who support the Order through taxation.

And then there's the Slavery question. Do the Knights use slaves on their plantations , do they tolerate other plantations on their islands using slave labor?

So, the Knight's sources of income are
Taxation from their island
Profit from their plantations
Donations
Selling pirate ships captured intact (Assuming they don't keep them all for themselves)