Add a sheet of nori to a blender and blend until broken down.
Add the hot water, soy sauce, and rice vinegar (if using) to the blender. Blend until combined.
Add the chia seeds to a small bowl. Pour the liquid mixture over the top and mix until uniform.
Cover and refrigerate for at least 2 hours or overnight, until the chia seeds swell into caviar-like beads.
Stir gently and serve chilled with your favorite accompaniments.
