Thinly slice cucumbers, preferably with a mandolin. With thick cucumbers, peel, halve lengthwise, and scoop out seeds before slicing.
Mix vinegar, ginger, sugar and salt. Toss with cucumbers and let stand for 30–60 minutes, or refrigerate if marinating for more than 1 hour in warm weather.
Just before serving, drain excess liquid. Heat 2 tbsp oil in a nonstick pan over medium-high heat. Sear scallops, about 2 minutes per side, then remove to a plate. Turn heat to medium, add remaining oil, onions and turmeric. Cook until onion softens, about 5 minutes.
Meanwhile, toast sesame seeds in a dry skillet over medium heat until browned and fragrant, 3 to 5 minutes. Stir onions into cucumbers, top with scallops, and garnish with sesame seeds to serve.
Shrimp can be substituted for scallops.