Dipping the tail rotor in the water would slow the main rotor down for a mili second at which time the tail rotor and gearbox would be torn away from the tail cone and the drive shaft would suffer a great deal of damage..
There was a case where a pilot mistook a river for the active runway and he set his H-300 down in the water. When the tail rotor hit the water the drive shaft wound up torsionally, severely shortening it and the diaphragm couplings stretched to make up the difference caused by the torsional shortening of the shaft. The diaphragm couplings then separated from the gear drive and the tail rotor gearbox and the helicopter came to rest on the river bottom. As an aside the pilot sued Hughes for the damage to the drive line..