Heat oil in a skillet over medium heat. Add sugar and cook until it begins to brown, stirring occasionally, about 3 minutes.
Add garlic and onion and cook until fragrant, stirring constantly, about 1 minute.
Stir in red pepper, fish sauce, and coconut water and cook for 1 minute. Add sardines and break them up slightly into large pieces. Cook, stirring occasionally, until slightly thickened and golden brown, about 5 minutes.
Season with soy sauce, green onions, and pepper and remove from heat.
Divide rice among serving bowls and top evenly with sardine mixture and sauce. Serve topped with cucumbers, cilantro, lime wedges, and more green onions if desired.
