true the ocean is your best bet for closeness, but you're more likely to find folks who board in fresh water.
if you want to board on a LAKE, castaic is closest at about 45 mins north up the 405/5. next would be pyramid a bit further up (another 10mins or so) that same trek or piru (1hr), also out that way.
after that, elsinore is about 1hr 15min drive (out east off the 91->15) and canyon lake is another 10 minutes past that (just a bit further down the 15), although canyon is a private lake. lake perris is out the 91->215 and about 1.5hrs out.