Cut in half lengthwise; remove seeds.
Prick inside with tines of fork; season cavities with salt and pepper.
Add 1 teaspoon each of butter, brown sugar, and water to each squash.
Wrap each half, cut side up, in 12x18-inch piece of heavy-duty foil; seal securely.
Place cut side up on grill.
Grill over medium coals till tender, 50 to 60 minutes.
Open; stir to fluff squash; sprinkle with additional brown sugar.