I don't want to keep banging on about my experience of ditching but....
We were in the Tay Estuary for 15-30 mins in October. I don't know what the temperature was, but probably between 5 and 10 deg.
My elderly parents were only in the water for 15 mins and suffered palpable shock, which my mother is only just getting over, and hypothermia.
The pilot and I were in the water for thirty minutes. He is small and thin and after about 20 mins was really suffering. I noticed signs that he was losing concentration, and between 20 and 30 mins was having to keep shouting at him and give him progress reports on the rescue just to keep him "with me." I am big and fat, so didn't feel the cold quite so quickly, but I didn't have the strength to get into the hovercraft, and on arrival in hospital, after 20 mins in a space blanket my temperature was 33deg.
I reckon that I am still feeling the after affects in terms of tiredness and loss of concentration.
And that was 30 mins in relatively warm water.
Get a dinghy, keep it somewhere you can get at it, learn how to get into it...or get another engine. IMO.
Will