Yet if you read about ditching a helicopter it does say to do exactly that.
A normal flare is performed, and then as the helicopter settles into the water the pilot decreases rotor RPM to minimum and uses lateral cyclic to roll the aircraft on its side. This causes the blades to strike the water and immediately stop turning, so that people are not hit by turning blades while evacuating from the aircraft. Most helicopters will sink fairly quickly since there are not normally large spaces to trap air in a helicopter. This makes it critical that everyone is briefed in how to get clear of the helicopter and get to the surface of the water.