Combine the soy sauce, sesame oil, 2 tablespoons ginger, 1 tablespoon garlic, and ¼ teaspoon red pepper flakes in a 1 gallon sealable plastic bag. Add fish and sausage to marinade, turning to coat. Refrigerate ½ hour.
Combine clam juice, broth, vinegar, and sweet chili sauce in a small saucepan. Boil over medium-high heat until reduced to ½ cup, about 10 minutes. Set aside.
Puree cilantro, macadamia nut oil, remaining 2 tablespoons ginger, remaining 2 teaspoons garlic, lime juice, zest, ¼ teaspoon salt, and remaining ¼ teaspoon red pepper flakes in a food processor. Reserve 2 tablespoons for shrimp and set remainder aside.
Preheat oven to 450° F.
Remove fish and sausage from marinade, scraping off excess, and place on a parchment-lined baking pan (sausage should be in a single layer on pan). Bake 7 to 9 minutes or until just cooked through.
Meanwhile, to finish sauce, reheat broth mixture over medium heat and stir in cilantro pesto.
Gradually stir in butter and season to taste with salt.
Melt reserved cilantro pesto in a large nonstick skillet over medium heat and sauté shrimp until opaque, about 1½ minutes per side.
To serve, divide watercress onto 4 warmed plates and top with fish. Drizzle sauce over the fish and top with shrimp. Garnish with tomatoes.