Protecting the Crystal
If you arrived from the western shore or explored the area around Florem, you may have passed by the Temple of Water earlier. To reach the temple, travel west from Florem city, over the first bridge, and then follow the trail south. Before long, you'll reach the temple.
Inside, the interior is basically the same as the Temple of Fire. If your party was hurt by the imperial troops along the way, you can speak to the Adventurer on the left to heal your wounds. Once you're ready, head north and enter the Altar of Water.
Beyond, simply run left and inspect the altar at the end and Sylvie will do her thing. With the crystal guarded from harm, leave the temple and return to Florem city the way you came. Yup, that was pretty straightforward, but get ready for some fighting when you're back.