Basically you dress to the temperature of the water, you have to consider capsizing! Obviously this means a change of clothes for the summer. In general yachting clothes do not protect much when you are in the water. Also they tend to limit your freedom of movement in a dinghy (or while swimming). Remember that for the same reasons you don’t use a life vest but a swimming aid. A dry suit has no thermal insulation, your survival depends on what you wear underneath. Also it depends on not tearing a hole in it, something that is hard to do on a dinghy.
I have followed the wind surfers trend and wear a titanium lined base layer under a wet suit (in my case neoprene hikers). In summer the titanium base layer is replaced by a lycra rash top and in between I sometimes use a polypropylene (fleece) base layer. A wet suit (or hikers) will still do its job when damaged and allow full freedom of movement. Then I have the option to use a spray top to prevent wind chill. To top it of I use a cap (summer) or a beanie, mainly to protect myself from the sun. And finally, when it is really cold, I may choose to use rubber kitchen gloves (supermarket) over fleece gloves (el cheapo from Lidl). On my feet are neoprene hiking boots, always, because they provide the best anti-slip and allow me to walk in the water while launching without getting cold feet. In winter a pair of titanium lined socks are available but I have yet to use them.
Andah, Colin, thanks for your pointer, these Rooster hikers are perfect for this garden gnome.