Serve this bright and refreshing seviche in scallop shells for extra flair.
Working over a bowl to catch juices, use a paring knife to slice between sections and membranes of grapefruits to separate segments; place segments in bowl. Squeeze juice from membranes into bowl. Strain; reserve juice in a bowl at room temperature. Place segments in another bowl; cover with plastic, and refrigerate.
Slice each scallop into three rounds; place in a large nonreactive bowl. Add reserved grapefruit juice, lime juice, and salt; toss well. Cover; refrigerate 30 minutes. Drain, discarding liquid. Refrigerate scallops until ready to use.
To serve, mound scallops and reserved grapefruit segments in serving dishes. Sprinkle with jalapeno and red onion. Spoon watermelon over the tops.