Into a blender, add watermelon seeds, cashews, and water, and blend until smooth. Pour mixture into a nut milk bag and squeeze to extract milk into a large pot.

Place pot over medium-low heat and slowly warm mixture, scraping bottom and sides occasionally with a silicone spatula. Curds will form and separate from the whey. Once most of the mixture has formed curds, stir in salt.

Pour mixture through a sieve or ricotta mold set over a bowl. Let drain 30 minutes to 1 hour, or longer in refrigerator until cheese has reached desired firmness.

Transfer to a storage container and refrigerate up to 2 weeks.