You'd best call the rangers there, it seems like the policy changes a bit each year, generally getting more stringent, but not always. We've had our boat in a number of So Cal lakes and it's always a huge deal trying to call ahead and figure out exactly what the inspection procedure is going to be and if we can pass-- our boat has hard ballast tanks, so removing isn't really an option.
We did not go to Castaic last year, but in the previous several years they didn't seem to worry about the ballast tanks, but they do open the engine cover and look in the bilge for any sign of water at all. Things (including toys, etc.) must all be completely dry.
We went to Piru last year instead, it has a more relaxed and mellow atmosphere than Castaic and we really enjoyed it. It has one arm of the lake where the kids can swim-- Castaic does not allow swimming in the main lake, only skiing. The inspection at Piru was probably even more rigorous than Castaic, but again for some reason the ballast tanks were not a concern.
Lake Nacimiento last year was the same story.
(I've read plenty of scary articles about having to bleach the ballasts or steam-blast with boiling water-- a procedure that requires an appointment ahead of time and some number of hours, but haven't actually run across it yet.) As I said this seems to be a bit of a moving target, so best to call ahead and find out for sure what the policy is this year as you don't want to arrive and be denied.