San Francisco weather in January

[saveyourlife]

Is the San Francisco weather in January pleasant enough to spend long hours outside? I understand the average weather would be approximately 55 degrees, but would like to know whether that doesn't interfere with enjoying a pleasant afternoon outdoors. Is a sweater and sweatershirt enough for being outside? Is being on a boat enjoyable in such weather, or is it too chilly for that?

Thanks for your help!