In this answer, I would like to list some anthropological and psychological factors that influence how peaceful people of different religions can coexist.
There are objective needs that must be met in order to survive and thrive. There are also cultural assumptions of how (and how certain) these needs will be met. These assumptions are formed by environment and religion. A worldview of abundance and love for all will be better at promoting peace than a view of scarcity and entitlement.
People tend to imitate their god. Gods lead by example.
Well the Meyra, another race, believe that the islands are sinful parts of pure mother ocean along with other Meyra who pure mother ocean believed so foul that they were cast from her embrace.
If their god casts out foul people, they will do the same. If their god throws away whole islands and the people on them, they will leave their trash there and treat the people like trash. On the other hand, if their god cares for the islands and the poeple, they will care for them, too.
Scarcity or Abundance
How do the people view their world? Is it a world of scarcity, or of abundance? Are those islands able to regenerate in a short time? What does their religion teach? Does their god provide for all, the hard-working and the lazy, the good and the bad, the true believers and the heathens? Or are the true believers entitled to take the scarce resources from the heathens, by war, robbery, or additional taxes and duties?
Omnipotence or Symbiosis
If there are many gods, your god might depend on your help for his own well-being, or to defend against other gods. You will never fully understand why he needs these sacrifices, but they are essential. Otherwise, he might not be able to bless and protect you. A symbiotic god is affected by scarcity himself. He needs you to provide for him. And to fight for him, against the unbelievers.
An omnipotent (or at least very potent) god can provide for you in abundance, and he can fight for himself. The unbelievers do not bother him. He will either convert or extinguish them, as he pleases, whenever he wants. It's not your job to fight them.
An omnipotent god might even command you to love your enemies and their islands (if he loves them and provides for them, too).
Man tends to emphasize the features of a religion that suit his needs and plans, and forget about the others--either willfully or by rationalization. This is a factor against peace. Your species might be different.
E.g., the Cellene live on the islands, and they get wood, food and lots of other stuff from the islands. When another people arrives and cuts the woods, hunts the game, and leaves a lot of waste, what will the Cellene do? They will defend their islands.
They might rationalize that it is their divine duty:
The Cellene believe that it is there job to care for these islands to the point they give up stuff from themselves
And they will always quote this noble reason. But they'd fight anyway, to protect their own livelihood and property. They might even start a war, to protect the other poeple's islands.
A god might be more explicit in what he deems good or bad. Or even intervene and remind his people of his commandments.
Ideas and Sources
Edward T. Hall postulates that all areas of a culture are connected to each other. There is a map of culture as a tool for anthropologists (and worldbuilders) at the end of his book "The Silent Language".
Kate McCord shows how culture is influenced by religion. In "In the Land of Blue Burqas", she relates how, by meeting a foreign culture and religion, she has learned as much about her own.