1. Place 4 cups watermelon in a blender and blend until smooth. Pour watermelon purée through a fine-mesh sieve into a large bowl. Add remaining watermelon, tomato, mango, cucumber, garlic, onion, lemon juice, and vinegar. Season with salt and pepper. Cover and refrigerate for at least 1 hour and up to overnight.
2. Just before serving, top with crab meat, a dollop of crème fraîche, and basil.