Disney's sea had to act, nod and high-five, and never look like a sea creature: "we really want to make sure this doesn't look like a sea creature," said head of animation Amy Smeed. Animators moved it "almost like a sock puppet", and effects filled it with water. Its feelings are timing, and real water looked "too agitated", so they turned the water down. You'd guess gravity wins, but while it acts it isn't falling at all: "Typically the simulation was run in a rest position, and bound to animation", with water streaming over the shape and forces sucking the sea up into it. The animators had animated the whole scene. So they let go on purpose. In head of effects animation Marlon West's words, "after the big beat of whatever performance, whether it's a high five or a head nod, we would stop using the character animation and then just let the physics, the natural physics of water, take over." The splash is how you know it was the ocean, and they agreed it would never pull its arm back in. Where it holds still, it's less real than water: when it parts for toddler Moana, Disney smoothed the walls and changed how they bend light, for "an aquarium-like feel, something non-threatening and beautiful."

The sea, the water column and the walls are our own particle recreations, not film footage.

Which movie trick should we take apart next?

Sources: Ben Frost, Alexey Stomakhin and Hiroaki Narita, "Moana: Performing Water", SIGGRAPH 2017 Talks (Walt Disney Animation Studios); Alexey Stomakhin and Andrew Selle, "Fluxed Animated Boundary Method", ACM Transactions on Graphics 2017 (Disney Animation); Marlon West and Amy Smeed on Science Friday, 23 Dec 2016; Marlon West to The Atlantic, 31 May 2017; Ben Frost in SideFX's Moana project profile, 31 Oct 2017.
Sounds: our own, made in code (earshot, CC0).