Thanks for joining us. I'm in NZ too (hope you and your family are keeping well).
Rats likely are the reason you are losing chicks. They are such a pain. If you can you could keep the hen and chicks confined to a safe run until they are a little larger.
It will depend on what duck species are available in Samoa and whether you want them for meat or eggs. Most domestic ducks, especially those used for their meat, don't fly and I don't see the tropical climate in Samoa worrying them. The most important thing for ducks is providing them with enough depth of water to wash the length of their beak and a small pond would be appreciated, especially when it's hot. Muscovy ducks might be another one to look at. They can fly but once they are full grown they don't tend to (especially the huge drakes). Their meat is apparently very lean and less fatty than Mallard derived ducks.
I'm just trying my hand at hatching some Muscovy eggs so I've still got a lot to learn, but there are lots of lovely, helpful people on the Duck forum who are always happy to answer questions.