I am freezing up here. Gonna be stiff as a board tomorrow, and not just because of the apres bateau pub barmaid's lovely biceps 
Keels - I meant more a defined midline, not yer actual keely keel. But I don't know what I'm talking about so I'll shaddap 
Most of the inland/river boats these days are pretty short - 2.3m or so for mine, and the hull shape is pretty flat with definite edges along the outside. When I first took mine out onto the sea (millpond calm) I spent quite a while getting used to the way it would suddenly catch an edge on an almost nonexistant wave and take off. The 'touring' boats and the much longer sea boats are definitely more V-shaped, but they turn the opposite way: If you edge left in a river boat, you go left. Do the same in a sea boat and you'll go right.
Learning curve? you bet.
Elleigh, what kind of weekend is it? I've only pottered about on lakes or in a calm bay, or done a tiny bit on a river, so I'm not exactly what you'd call expert myself! I'd second Andy's note, and add: A dry liner from a pannier is useful (but don't keep it in the boat unless it's
really watertight. Some boats have bottle holders for bike-style water bottles, so you can carry a drink. I found getting wet wasn't a problem, as long as I didn't get cold - a steel flask with a hot drink can work wonders at lunchtime, as can flapjacks and Mars bars. If you're on a course and everyone's itching to get out on the water, don't be put off spending a little extra time getting the boat fitted to you properly - if you have trouble reaching the footpegs/footplate, you won't paddle as efficiently. Hip pads may need to come out (I don't use them in mine) and the backband may need adjusting. Take the time to make it a snug fit and you'll be much more comfortable on a long day.
