Those are good points and definitely worth repeating.
I happened to go out to PaG today. It smelled like absolute hell out there. Tons (literally) of Chaetopterus worms, (this is a pick of the worm inside those tubes) http://flickr.com/photos/artour_a/1520309708/ , along with sea urchins, pen shells, scallops (anything on the bottom) washed up on shore. Man... it was nasty. I don't remember seeing anything quite like it. It was solid crap along the entire shoreline.
Not so good for tourism.

In terms of beaches to practice, I would actually recommend Treasure Island over North Beach. North Beach can be pretty crowded on weekends starting about this time of year. Jim prefers if we just avoid that beach starting around now for bird nesting reasons and because lots more beach goers start showing up. It's not a rule per se, but don't be surprised if when out there you are approached and politely asked to leave. If so it's probably pretty obvious to not ask to see the sign or argue about it (that'd be a really bad move).

TI is a good alternative because it is huge (wide) and the area to the south usually isn't too crowded with people.