Bring the water to a boil and stir in the sea salt to dissolve. It's normal for there to be a bit of sediment when using mineral-rich sea salt. Allow it to cool until it's tepid.
Add the sliced jalapeños to a quart-sized mason jar. Add the garlic, if desired.
Fill the jar with brine, leaving enough headspace for the weight.
If they're available, cover the jalapeños with grape leaves, tucking them under the shoulder of the jar to help keep them submerged. The tannins in the grape leaves will also help to keep the peppers crisp.
Follow the jalapeños with a fermentation weight. If you don't have one of these, find a stone (wash it first!) or fill a small ziplock bag with water.
Place an airlock over the jar and close the jar. If you don't have an airlock, close the jar, but open the jar every day to release any gas that might be building up inside.
Set the jar on a baking sheet to protect against overflow, and allow it to ferment for the next 4-7 days. When the peppers have a tangy bite, remove the weight and grape leaves and transfer to the fridge. The jalapeño pickles will keep for a year.