What it's mostly about is responsibility. Moana, daughter of Cheif Tui (Temuera Morrison) and Sina (Nicole Scherzinger), longs to sail "beyond the reef," but Tui has forbidden any of the villagers from doing so. Her parents continually chide her for her oceanic fascination and try to guide her to accept her responsibilities as the chief's daughter.
When the crops fail and the fish fail and even the coconuts fail, Moana, urged by her dying grandmother (Rachel House), sneaks off to sea to find the demigod and set right an ancient wrong that is blackening the world. Well, the demigod has an ego the size of the Big Island, and all he's interested in is getting his magic fishhook (which lets him change shape) back. But the Ocean (very much a character in itself) has chosen Moana to perform this quest, and it isn't putting up with any guff from a demigod. He has to learn to take responsibility for his actions, and he does.
Some of the songs are brilliant, like Maui's "Thank You" and the hilarious "Shiny" (sung by the coconut crab Tamatoa (Jemaine Clement). Some are not so much, but none of them are bad. There's a Broadway musical just waiting to be adapted here. And the soundscape is simply wonderful.
All in all, a B+/A- film on the Disney scale.