The Rise: Dystopia. The Rise Trilogy, #1

The entire planet is forced to continually fight for higher ground as an unexplained, unrelenting rise in water covers the earth, destroying infrastructure and isolating communities. The resulting dystopian society forces alliances between unlikely groups, brought together by nothing more than desperation and geography. As food and water dwindle and tensions rise, generous relief supplies are provided by an anonymous source.
The sporadic deliveries, which seem too good to be true, eventually create suspicion among the survivors who speculate about the origin and motives of the mysterious donors who provide them.