Now group B decides not to deliver any water to group A. In fact, group B decides that since they don't want to waste their water on the infidels of group A they would much rather just wait untill they all die out.
You make a lot of assumptions and emotional appeals. But no, group B cannot be forced to deliver water to anyone. They can trade it voluntarily with others.
The logic is that science allows us to check the premises of our gut feelings of ethics.
It makes ethics more rational.
With universally preferable behavior the scientific method is used to come to a rational framework for ethics. This also allows to check the "gut feelings of ethics".