I've deliberately avoided this post, but curiosity has got the better of me - so here's my 2 bobs worth...
There have been lots of suggestions and opinions offered ranging from the downright sensible to the pretty wierd.
The immersion suit with lots of warm stuff underneath is a must, as is the good quality lifejacket (not the self-inflating type) with spray hood. A reliable SARBE/PLB is also a must-have. A dinghy? Well of course.... No mention of floats so far, so I guess that you won't have them.
If you do have to ditch and assuming that you pull off a perfect touch down in a benign sea-state, you have get yourself and your "little life raft to keep the CAA happy etc." out of your helicopter which will have the bouyancy of a punctured crowbar. Have you done a dunking course? If you haven't and it comes to the worst, you will be doing it for the first time for real. Not an ideal arrangement.
Now, we all know that engines can't tell that they're over water, and as you say "But ya know some times engines don't stop!" Engine failure is only one reason why you might end up in the water.
If you have to do this trip, plan for the worst - don't hope for the best. Take the shortest overwater route and only in the best weather.
I did this trip once in a 206 without floats. It was summer, we had the suits, jackets, dinghy, SARBE, flares, you name it. Precisely one mile short of half-way, the engine chip light came on. Conversation dried up rather as we headed for Valley (it wasn't the shortest over water route). We made it, but although the chip was the size of an ants eyelash it could have had a fully legible part number on it for all we knew.
Forget about landing on the ferry, they'll NEVER let you do that.
Good luck, let us know when you've done it.