Mix water, calamansi, salt, and pepper together in a bowl until well combined.
Brush the brine mixture onto cleaned fish and leave to brine for 15 - 20 minutes.
In a bowl, mix butter, turmeric, grated ginger together until well combined.
Brush the butter sauce onto both sides of the fish.
Place the vegetable skewers onto a grill and cook until charred, about five minutes.
Grill fish for about 5 minutes each side then remove fish from the grill.
Wrap grilled fish in banana leaves and grill for another 10 minutes each side.
Remove vegetables from the skewers and saute in a pan with oil over medium heat for a few minutes.
To the pan of vegetables, add patis, coconut milk, and lemongrass and reduce until thick. Pour over grilled fish.