No, a single 20-ampere circuit cannot supply power to both the 600-watt underwater light and the 8.5-ampere circulating pump. The total current draw of both devices is 8.5 + (600/220) = 11.73 amperes, which exceeds the capacity of a 20-ampere circuit. To supply power to both devices, two separate circuits are required. The underwater light can be connected to a 15-ampere circuit, while the circulating pump can be connected to a 20-ampere circuit. It is important to ensure that the circuits are properly sized and wired to meet electrical safety standards and prevent overloading or overheating.