Place potatoes in heavy large saucepan. Add water to cover. Season generously with salt. Bring to boil. Cover and cook until potatoes are tender, about 25 minutes. Drain. Cool slightly. Peel potatoes. Cut potatoes in half lengthwise, then cut crosswise into 1 cm thick pieces.
Place half of potatoes in large bowl. Drizzle with 1 tablespoon vinegar. Season with salt and pepper. Add remaining potatoes. Drizzle with 1 tablespoon vinegar. Season with salt and pepper.
Melt butter in heavy large skillet over high heat. Add prawns and scallops and sauté until cooked through, about 5 minutes. Transfer prawns, scallops and any juices to bowl with potatoes; add crabmeat. Toss gently to blend. Cover and refrigerate until well chilled, about 2 hours. (Can be prepared 1 day ahead. Keep refrigerated.)
Stir sour cream, mayonnaise, spring onions, minced tarragon and remaining vinegar in small bowl to blend. Pour dressing over potato mixture. Toss gently to blend. Season to taste with salt and pepper. Garnish with tarragon sprigs and serve.