Ask the fishmonger to cut the grouper into 1-inch cubes for you and remove the shells and veins from the shrimp. You’ll want to purchase clams that are the same size so that they cook evenly. Adding the clams to the stock while it comes to a boil helps speed up the process.
Soaking the skewers in water prevents them from catching fire during grilling. If you have metal skewers, use those instead.
Serve with plantains, refried beans, and red onion escabeche.