Mix sliced zucchini, onion and herbs and tightly pack into a clean, sterile 16-ounce glass jar until about 3/4 full.
Dissolve sea salt in water to create brine solution. Stir until salt is dissolved, then add vinegar, if using.
Pour liquid into jar to cover zucchini and herbs. Add more filtered water if necessary so you only leave about an inch of room at the top.
Close the jar and leave on your counter for 3-7 days (or longer if you prefer) until naturally pickled by fermentation. Unless you are using an airlock lid, loosen the lid a little every two days to let built-up carbon dioxide out.
Or, if using vinegar for fast pickles, refrigerate for about an hour until chilled. (Better if left to pickle longer though.)