It will not be that cold I think. It could reach freezing point at the very lowest, pre-dawn temperatures, but during the day you will probably be more in the range of 8-12° Celsius. Overcast quite probably, sometimes rainy as well, but unobscured sun in a claer sky doesn't make for the best lights, as we all know... I'd pack one warm dressing option, but a couples more "medium" ones.
As regards photo ops, it will depend on the boat schedule: at what times of day will you be sailing? Through what kind of countryside? Where will the light shine from then? With the help of internet resources (Google Earth first and foremost) and a good photo app such as Photographer's Ephemeris on the iPhone, you can really plan a photo trip very easily nowadays.
You will basically be shooting in two types of situations: from the boat, as the countryside rolls by, and on terra firma, during stops. By studying carefully when each of those situations will happen, you will be able to plan some shots in advance. Of course, there will always be an element of surprise, and that's maybe when you will take your most prized photos...!