Preheat the oven to 400°F (200°C).
Place the salmon fillets skin-side down on a large sheet pan spaced evenly.
Arrange the sliced yellow, red, and green bell peppers and the sliced onion around the salmon on the sheet pan.
In a mixing bowl whisk together the fajita seasoning packet, lime juice, minced garlic, olive oil, and water until combined.
Spoon the mixture evenly over the salmon fillets, then pour any remaining mixture over the peppers and toss the vegetables lightly to coat.
Bake in the preheated oven for 15–20 minutes, until the salmon flakes easily with a fork and the peppers are tender.
Serve the salmon and peppers with fresh lime slices and chopped cilantro as garnish.