The SF bay is a wonderful place to sail for many reasons, not least of which is that depending on where you are you can experience a wide range of sailing conditions, both heavy and light winds, heavy and light swell, cold and foggy, warm and sunny - all in the same day if you wish!
As you have discovered, summer ain't winter. Take in the Genoa, break out a 100% jib, learn to reef - it gets windy in summertime....
Use Sailflow (SailFlow.com - CA-sf south Wind Data
) to check the wind patterns. L-36 also has an awesome weather page (I use Redwood City, CA Marine Weather and Tide Forecast
Most of the time I sail in the south bay. There are two main considerations: Time of day, and tides. Typically in summer the wind builds daily from 5 knots in the morning, to 20-30knots around 4-5pm, and then it gently winds down.
The other factor is tide, The wind generally blows down the bay - ese. If the tide is coming in, the waves are minimal - typically around 1'. If the tide is ebbing - against the wind - the waves quickly build to 3', sometimes higher. In a small boat you will get wet.
Around San Francisco, the city (and its headland) often causes a wind shadow, and that can be fun, but beware - as you head north you get into "The Slot", where the wind comes through the Golden Gate bridge into the Bay proper. I have had to go from a full main/headsail to (within a couple of boat lengths) double reef, no jib, still healing heavily as I race to Angel Island. Then as I exit the slot on the other side (Tiberon, Sausalito, etc) out come all the sails again!
Once again, always look at the tides; if it going against the wind, the wave builds fast.
Hope this helps....