If it's not coolant than I would suspect that the water got in there while cleaning the engine if you did that recently or if you drove through a big puddle.
If it's coolant then check your intake crossover in the front of the engine and the heater bypass that runs just above the rear of the intake for leaks.
If you clean your COP boots thoroughly and spray them down with silicone spray before installing they will seal much better and prevent water from getting in the plug well.
It is critical that you determine if that is coolant or just rain water.