r13 said..
Thanks a lot yes agree regarding using a lashing able to be severed with a knife - I will do that. Lashing strength equivalent to the wire strength.
Add a ceramic knife taped to a stanchion on the transom at deck level, it's much easier than trying to get a knife off your belt. Leave a long enough loop, a few inches that you can easily cut with a knife as you're most likely to be one handed/half drowned/being dragged. I was surprised that soft lashings never required redoing to keep everything tight, the normal wire turnbuckles comfortably take up any stretch that creeps in.
The other benefit is when you cut the line, the upper safety line ends up being a wire trapeze line you can stand on. The turnbuckle stops the safety line at the next stanchion, so now you have a nice big loop of wire you can stand on underwater forrard of the next stanchion.

I had the same set up across the transom as I had a post (not a stanchion really as it was removable) in the middle of the transom. The approx 2mtrs (transom) to 2.5mtrs (sidedecks) of slack gets your shoulders well above the deck when getting back on.