The Water Dragon’s Scale is one of the Goddess’s three gifts. It grants the user the ability to freely swim underwater and perform a spin attack.
While underwater, hold the A button and move the Wii remote/Joy-Con/right stick to swim around. If you release the A button, you’ll start to float upwards and you can move with the left stick instead.
To do a spin attack, shake the nunchuck/Joy-Con/click the left stick inwards. This will let you destroy fragile objects or hurt monsters. If you aim upwards near the water’s surface, you’ll spin through the air–you can use this to climb up ledges too high for your arms to reach.