In a small saucepan, heat 1 tablespoon of the olive oil until shimmering. Add the apple, onion, curry and saffron and cook over moderate heat until the onion is softened, about 5 minutes. Remove from the heat and stir in the water; let cool.
Scrape the onion-curry mixture into a food processor. Add the mayonnaise and process until smooth. Transfer the curried mayonnaise to a medium bowl, add the cilantro and mint. Gently fold the crabmeat into the curried mayonnaise and season with salt and pepper.
Cut each slice of watermelon into triangles and transfer to plates. Season the watermelon with salt and pepper and sprinkle with 1 teaspoon lime juice. Mound the crab salad on top of the watermelon triangles.
In another bowl, toss the arugula with the remaining 1 tablespoon of lime juice and the remaining 2 tablespoons of olive oil and season with salt and pepper. Place on top of the crabmeat and serve.