Super King Air 300
Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 100; Flight Crew Total 8600; Flight Crew Type 500
Workload; Situational Awareness; Physiological - Other; Fatigue; Time Pressure
Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Loss Of Aircraft Control; Ground Event / Encounter Weather / Turbulence; Ground Excursion Runway
Flight Crew Regained Aircraft Control
Environment - Non Weather Related; Weather; Human Factors
Shortly after touchdown on Runway XX the plane began drifting to the left; controllability was deteriorating rapidly. The SIC was attempting to correct the drifting; I (the PIC/PM) stated we're drifting left and applied Full; Authoritative; Firm pressure to the right rudder.  The plane began to straighten out and correct right. The left tires were on the edge of the runway next to the grass and water runoff; causing hydroplaning. As the plane decelerated I was able to regain control and corrected back to the center of the runway. The SIC further assisted in maintaining control of the aircraft as we could feel the tires slipping and hydroplaning. With team work and crew coordination; we regained full control of the Aircraft again and we're able to stop and clear the runway onto a taxiway.Contributing factors include; but not limited to: Weather; rain; winds/crosswinds; darkness/night; unfamiliar airport; wet runway; uncontrolled airport; fresh grass clippings and water pooling along edge of runway; long duty day greater than 12 hours;  consecutive long duty days; multiple flights in a duty day (4th);  human factors; pilot/crew exhaustion; crew dehydration due to extreme heat and humidity throughout the day; high stress loads throughout the day; get there itis; external pressure from passengers;  crews desire to succeed and make everyone happy by completing the mission.
After touchdown the aircraft began to drift to the left. Control correction was applied as the aircraft was drifting but with very little effect. As we neared the left edge of the runway the Pilot Monitoring (PM) assisted the Pilot Flying (PF) and applied full right rudder.  Due to the water collecting on the runway the aircraft did not respond very well to the rudder input until we decelerated.  At that point we had drifted slightly off the runway edge onto the grass. With crew coordination we were able to recover the aircraft back to the runway center line. We excited the runway without further incident.Contributing factors:Weather; fatigue; hot temperatures and humidity throughout the day; dehydration; lack of nutrition; end of a long duty day (arrived at ZZZ on hour 13) preceded by multiple long duty days.This event has opened my eyes a little wider to the complexity of risk assessment. In the future more emphasis will be placed on assessing external pressure; fatigue; and physiological needs.  Also more assertiveness towards these areas of risk assessment. ADM is a very complex system of checks and balances and awareness to even the slightest deviation can prevent future contributing factors to this and other events. A lot could be benefited from a personal assessment matrix to pair with a flight risk assessment matrix to help with mitigating a go/no go decision.
The crew of a King Air 300 reported loss of control during landing but were able to regain control. The crew cited weather; including wet runway and cross winds; also sighting fatigue and a long duty day as contributing factors.