You can just show up and run within the times as set forth by SF Park & Rec. and prōclāmāted thereby in signage nailed upon a tree...
Basically, 10:00am to 1:00pm for power boats; Sail from 1:00pm to 4:00pm. Nitro-powered boats are restricted to, IIRC, Tuesday, Thursday and Saturday, as well as being covered by noise and speed restrictions, again, Park and Rec Dept. rules. Spreckles Lake itself is always open to the public.
If there is a Regatta, you can join the Regatta if you wish or not, you just have to abide by the Frequency board (have the Freq flag for your transmitter unless you're using a 2.4ghz like a Specktrum).
The Navigator Regatta is a Navigation/boat handling skills contest, but I'll leave the specifics to either CaptLee or Aerominded to tell since they're a bit closer to the planning than I. It might be somewhat like this Tug Event
back in '06.
Looking forward to seeing your boat and meeting you on The Lake!!
ps. Got pictures of your boat??? Inquiring minds wanna see