The Canteen is located in Fort Sam Clemens at the far end of Tom Sawyer Island. Let me just say right off that the milkshakes at The Canteen are so good that even if you have no desire to visit Tom Sawyer Island it's worth the trip just to get the milkshake. Yes, they are that good.
They also have a few snacky food items here to feed the appetites of hungry explorers. There is no seating and very little shade in the immediate area of The Canteen. What many people do is get their food and/or drink, walk up the stairs into the fort, find one of the many alcoves inside the walls of the fort, sit on a step and enjoy the food and/or drink.
If you're visiting Tom Sawyer Island certainly do not miss a chance to stop at The Canteen.