The jolt could have come from several places. Perhaps the water rather than the microphone was electrified (as by leakage from heaters that keep the baptismal from getting cold). The microphone then being grounded provided the current path. Does the baptismal have underwater lights like a pool? Another source of current leakage. Someone using a cheater plug to plug a three-prong amplifier plug into an old two slot outlet, combined with electrical leakage existing in the amplifier's power supply. Or the amplifier plugged into a power outlet that has the ground prong wired to the hot side of the line. (Not even a GFCI can help in that situation.)
The person that was to be baptised didn't get shocked. If they were already in, wouldn't it have got him/her too if it was the water.
...or perhaps the baptismal pool was in the spin cycle.