Put rack in oven in middle position and preheat to 475.

Stir together mayonnaise, chipotle and salt in a bowl.

Pulse 1 1/2 c corn in food processor with water and 2T of the chipotle mayonnaise until coarsely pureed. Transfer mixture to a shallow 2 quart gratin dish. Stir in scallions and remaining corn.

Toss shrimp with remaining mayonnaise in bowl to coat.

Put 1 cup of panko on a plate and coat each shrimp in panko crumbs. Arrange shrimp in one layer on top of corn. Roast until crumbs are golden and shrimp are just cooked through, about 18 minutes.