Are the instrument panels in the cockpit spill / waterproof?
No they are not, at least not fully. There have been instances of equipment failure caused by coffee spills, resulting in the need to abort the flight.
This example shows a flight that needed to divert because a coffee spill disabled the communication equipment.
This Boeing 707 had to make an emergency landing after the crew inadvertently spilled water on the autopilot panel causing the stabilizer trim wheel started to rotate. Control was lost as the plane pitched up and down. Three people died in the accident.
This Boeing 737 report had smoke in the cockpit because the previous flight crew spilled coffee and sugar over the avionics.
This Boeing 720 suffered control problems (resulting in Dutch roll) after a coffee spill cause a short circuit in the yaw damper.
This Airbus A330 diverted for an emergency landing after liquid was spilled onto electronic devices in the cockpit causing the smoke.

Source: Transport Accident Investigation Commission of New Zealand