Thanks for all the replies, it has certainly clarified things for me. It would be cheaper tbh, as I am about to buy my first boat, to buy a new boat, although at my stage of training I am in the wrong place, and it would be too expensive.
If I remember correctly the Greenline engine is bigger, and then there is the amount of batteries, and fitting as many additional solar panels as possible. I don't have a car anymore, but a few bikes, so that will be a green contribution, and the health spin off as well.
It's an interesting conundrum and something that will become more common till things change at some stage. That is a concern, but more for the younger people coming through. The younger are deadly serious about the ecology of things. The one boat I looked at, did not use coal, but composite pellets and blocks. There are organisations now that do not use bleach in domestic cleaning, and all that will be come more noticeable in time in every aspect of our lives. In time, we will just not be able to buy certain things.