I went inadvertent IMC in the Bahamas, over the water, in a 206 with no AI or turn coordinator. We were flying a nurse and patient back to Nassau from a cruise ship, picked from an island. Weather was cavok with the exception of a line of rain ahead of us. It looked like light rain, I could clearly see Nassau, the horizon, even sun in the water on the other side. Being in a hurry I decided to fly through it, and the moment we entered the rain it was like flying into a waterfall...nothing but water to be seen. I transitioned to whatever instruments I had, and tried my best to keep altitude, airspeed and HSI from moving, making small inputs....left cyclic 1-2-3 center....right cyclic 1-2-3...aft cyclic...etc...talking to myself to try to keep calm. All this time it felt like I was sitting in a chair tumbling backwards. I was terrified and I had to fight to keep panic at bay. I made a conscious effort to not move anything but cyclic, and to trust instruments. What really freaked me out was blade slapping sounding like a steep turn, with nothing on the instruments showing a turn. No idea how long it lasted, but we came out into sunshine about 400' lower, and in a 15 degree right turn, slightly nose low.
I'm sure that without my ifr ticket I would have been just another statistic, and would have been another one of those "idiot pilots that killed his passengers", but that rain looked like I would be through it in 2sec tops. Easy to judge with hindsight of course. At present I fly S-76s, frequently IFR, and I'm still very leery of rain.