Citation Jet Captain reported hydroplaning and loss of directional control followed by an excursion into the grass as he attempted to turn off the wet runway.

Narrative

The conditions [at destination airport] were VFR and it had just rained. The weather conditions at the time of arrival were 220@6KT 10SM 16/12 29.65' hg with Runway XX in use. The runway surface was wet as it had just finished raining before arrival. The calculated landing conditions with a wet runway surface were 2986 ft. and we had 4007 ft. available. The landing progressed normally and without incident.After touchdown during the rollout as I was turning off the end of Runway XX onto [the] taxiway the airplane started to turn normally; however as we were entering the turn the main wheels started to hydroplane. I released and depressed the brakes multiple times in an effort to stop the hydroplane; however was unsuccessful. When it became apparent we would not successfully complete the turn; I steered the front of the airplane toward the gap between the runway end lights and first taxiway light. The airplane slid off of the taxiway entrance of the runway between the two lights and came to rest about 8 feet outside of the taxiway between the two lights. The main wheels were stuck in the mud; the nose wheel was free and there was no damage to the airplane or airport surface.After getting the airplane out the mechanics noted that our main wheels were near the end of their service life and brake pads were worn and needed replacement. They confirmed that there was no damage to the airplane and released it for service.