ACFT experienced a tail rotor drive shaft failure during T/O from an offshore platform. ACFT spun left and completed 6 to 7 revolutions prior to water contact as the single PLT could not reach the throttles to reduce PWR to idle. ACFT landed hard and the RT float deployed on touchdown while the left float did not and the ACFT rolled left. Occupants swam to rig from which T/O had been initiated. Investigation revealed that the forward shaft (No. 1 segment) had failed in the area of the aft splines near the main gearbox oil cooling fan connection. Failure was attributed to fatigue due to fretting caused by abnormal vibration levels induced by the oil cooling fan blades. Analysis indicated that fan blades would become contaminated with deposits of grease, oil, and soot causing vibration to increase from .6 IPS to 4.0 IPS. No previous requirement to clean the fan blades existed and the vibration would not have been detectable by the PLT or PAX.