Meanwhile, blend jalapeños with water in a blender or with an immersion blender until thoroughly blended. Add diced cucumber and blend until pureed. Add lime juice and cilantro and blend until cilantro is finely chopped. Season with salt and pepper.
In a mixing bowl, toss scallops with jalapeño-lime marinade, onion, and remaining cucumber half-moons. Season once more with salt and pepper and serve right away with tostadas and avocado.