In general, layers are advisable so that adjustments can be made when rowing at different levels of intensity and in cold or variable weather. From the thighs upward, requirements are likely to be very similar to the clothes you would be comfortable in when cycling or jogging in any given weather.
Due to the need to wade into the sea to launch the skiff however, there are additional considerations for the feet and legs.
Each person is likely to find a combination of solutions which work for them depending on the conditions of the day, their body and personal preferences, e.g. how hot or sweaty they get when exercising, whether they will be coxing later in the day (and therefore need to avoid getting wet/cold at all), how prone they are to feeling the cold. Therefore recommendations for Summer may apply to some people quite late into Autumn.