In a stock pot or soup pot, bring water and sea salt to a boil over high heat. Add the squash and cook until soft, about 12 to 15 minutes. Strain and set aside. In the same pot, add the butter and melt over medium heat. Add the garlic, onion, cumin, and allspice. Sauté until the onion is tender, about 10 minutes. Puree the squash with the onion mixture and the chicken stock using a food processor, blender, or hand blender. Pour the puree back into the saucepan, add the milk, and bring to a boil. Reduce the heat and simmer for about 10 minutes, stirring occasionally. Season with salt and pepper and stir in the chives.