In a bowl, cover the macadamia nuts with the water. Cover with plastic wrap and let stand at room temperature overnight.
Pour the nuts and water into a blender and puree. Strain the nut milk through a fine sieve into a bowl, pressing on the solids to extract as much milk as possible.
Rinse out the blender and return the strained milk to it. Add the lime juice and agave nectar. With the machine on, add the xanthan gum and blend until the cream is light and airy, about 1 minute. Scrape the cream into a bowl and season with salt.