The weather in San Francisco is hard to predict.
September is generally a mild month,
usually the nicest of the year, with warm temperatures
during the day (70's to 80's)
and cool nights (50's to 60's).
Since the reception will be out on the waters of San Francisco Bay, temperatures may be somewhat chilly.
Layers are generally your best bet, that way you can regulate your clothing to suit the situation.
For the current local weather, you can check with
The Weather Channel