Line a baking sheet with a silicone baking mat or parchment paper.

Stir pumpkin seeds and sea salt together in a bowl.

Stir sugar, corn syrup, and water together in a heavy-bottomed saucepan over medium-high heat. Add butter and bring to a boil, whisking constantly. Boil until mixture is dark golden, 10 to 15 minutes. Remove saucepan from heat; whisk in baking soda.

Pour pumpkin seeds into sugar mixture and stir until well-combined. Transfer pumpkin seed mixture to prepared baking sheet. Place another silicone baking mat over the top of the pumpkin seed mixture and press down to make an even surface for the brittle. Let harden for about 30 seconds; remove top baking mat. Score surface with a knife to form desired shapes. Cool to room temperature and break into pieces.