First, I see Suiiki as a story about how ephemeral life can be, about how certainties can be overturned given enough time (or money), and about how the past is a story, the future is a mystery, and the present is a gift, and that's why we call it "present". (Phrase directly from Kung fu Panda) We see the part about life being ephemeral with all the supernatural and spiritual stuff involving Sumio and the supposed dragon god. Sumio was a child, but he passed away, leaving his parents only hope that one day they could meet again. It was a simple slip-up (almost literally) and Sumio wasno more.

We see the part about certainties when the people of the village, aware of the construction of the dam, unite and say with conviction that they will not give up. That they will fight to the end.
But we live in a world where certainties, made of steel, are overturned by paper. Paper that has a value capable of making any human being give up his home, where he has lived all his life, as long as it means giving his family a better future.

And talking about the future, we come to the maxim about time.
The mangaka masterfully treats readers to experience the same as Chinami, the protagonist:
At first we were confused about the past, about history, in the same way that Chinami was. Where she was? Who was that boy? Why was it raining non-stop in the middle of a dry season?
Over time we understood that that place was what the future held for us. The mysteries were all there, ready to be solved. With death, perhaps we could inhabit that place forever.
But before that, we needed to live the present. The present where the gift of finding something dear, lost for a long time, made living continue to make sense.
And it is in the figure of Kiyoko, Chinami's grandmother, confusing the reality of the present with the history of the past, that both Chinami and us, readers, realize that all that still lives in the inhabitants of the village.
And so, we ended up nostalgic, as if we had lived there too.