The second episode of “The Lord of the Rings: The Rings of Power” centers on an epic storm in the Sundering Seas. Pulling it off involved a method that has become Hollywood’s go-to approach for creating storms at sea: filming the sequence in a giant water tank. Outdoor water tanks have become a staple of productions big and small seeking to recreate scenes of violent ocean storms — from Kumeu Studios in New Zealand, where “Rings of Power” filmed its storm, to Pinewood’s horizon water tank in the Dominican Republic, where films like “The Lost City” (2022) and “Old” (2021) have shot aquatic scenes. Tanks allow filmmakers to exercise exacting control over the conditions of a seemingly chaotic scene, whether a tempest like that in “The Rings of Power” or a tidal wave like that in “The Impossible” (2012).