I've tried this on the Altis map only and only with the ghosthawk, humingbird and Huron so far, will test all the other helos and also the other maps and will report in with more info about the other vehicles
I've tested most other helicopters at the moment and almost all helicopter shadows didn't draw properly
MH9/AH9 rendered correctly
MH900 didn't seem to cast a main rotor shadow
CH67(black/normal) rendered both rotors but no hull shadow
AH99 only seemed to cast a shadow of the anti-torque/tail rotor
all other choppers didn't cast a shadow on the water at all.