Every year, like clockwork, on 4th of July, the Hermosa Beach Ironman competition is waged near the Hermosa Pier. There are no official rules posted in one location anywhere (I think) but here are some I found on the web (if you can trust the Internets):

- no official start time, people start gathering around 7am, but sometimes it does not start until 10am, registration is 9-10am?
- meet at 29th Street and Manhattan Avenue (though some sites claim that there is no set meeting place, it changes from year to year)
- it says somewhere that it costs $20 to participate (main prize is a donation in winner’s name to local charity)
- one mile run on the beach,
- paddling a mile in the ocean (must bring own longboard - no paddleboards)
- drink 6-pack of beer immediately after the physical exertion (bring your own beer, must be in cans)
- first one to complete the triathlon run/paddle/drink is the WINNER (yeah!)

200-300 participants and many many looky loos

These sites seem to have the most info:
http://www.southbaychat.com
http://www.southbayevents.com
http://ironman07.com

.. and btw, another several hundred thousand people also come to Hermosa Beach to celebrate 4th of July. Try not to drive because chances are that you will not find parking.