Wash the sea buckthorn berries thoroughly, removing any stems or leaves.

In a large pot, combine the berries and water. Bring to a boil and then simmer for 10 minutes, mashing the berries gently with a spoon.

Use a fine mesh strainer or cheesecloth to strain the cooked mixture, extracting the juice into a clean pot. (A food mill also works well for this, and can be used with raw or cooked berries.)

If desired, add sugar (if using) to the juice and stir until dissolved.

Bring the juice back to a boil, then reduce the heat and simmer for another 5 minutes.

Carefully pour the hot juice into the prepared jars, leaving about 1/4 inch of headspace at the top. Wipe the rims clean and seal with the sterilized lids.

Place the filled jars on a rack in a large pot of boiling water or a water bath canner. Make sure the water covers the jars by at least an inch. Process for 10 minutes if you're below 6,000 feet in elevation, or process for 15 minutes if you're above 6,000 feet.

Remove the jars from the water bath and let them cool completely. Store in a cool, dark place for up to a year.