Grease a baking sheet or line with a silicone mat.
Cook and stir peanuts, sugar, and water together in a wide, heavy skillet over medium heat until liquid evaporates and nuts begin to crust with the hardening sugar; about 5 minutes.
Reduce heat to low and continue to cook and stir, scraping the bottom of the pan to coat peanuts with syrup as the sugar begins to melt and darken to a deep caramel color, about 10 minutes. Tilt pan away from the heat as needed to allow peanut mixture to darken without burning. Remove from heat immediately when peanuts are deeply bronzed; stir.
Sprinkle peanuts with sea salt and cinnamon or chili powder; tilt peanuts onto prepared baking sheet to cool completely. Break up clumps. Store in an airtight container at room temperature for up to 1 week.