There's only one table service restaurant in Epcot that has a view of Illuminations: Rose & Crown. You can make a reservation for dessert there, but you can't reserve an outdoor table, it's just the luck of the draw as to whether one's available when you arrive.
An alternative would be the Cantina de San Angel. It's counter service, but you could grab churros or some other snack and sit at a table by the water. Be warned, through, that folks stake out those tables a LONG time in advance.