Trackpad interaction without touching it since spilling water
The basic working of a trackpad is when a charged object such as a the human finger interacts with it. In case of a water spill, I assume you have wiped and cleaned it but still some amount of moisture has managed to slip into the trackpad and when it gets charged it starts interacting with it. I would suggest to take it to a repair shop, or if you still want to test it you can connect the external trackpad and check as that should work fine.