If you'd like to maximize your dive time in the extraordinary habitat of the giant kelp forest, I would strongly recommend a multiday liveaboard in "The Channel Islands".
One-Day boats generally have to choose a specific island due to travel time.
90% of one-day boats go to either Catalina or Anacapa / Santa Cruz Islands.
Multiday boats will name an island but then have time to move you around based on conditions.
My top two choices, based on hundreds of boat dives in the past 8 years in SoCal, are:
The Peace, leaving from Ventura Harbor.
The Horizon, leaving from San Diego.
I know the Horizon has a 2-day trip departing August 5th at midnight, and diving the 6th and 7th at San Clemente Island.
Catalina has some gorgeous spots (Ship Rock and Bird Rock, Long Point, etc.)
But it also has some less than great spots.
If it's a one day, open boat, it's a roll of the dice.
San Clemente is usually extraordinary, in my experiences.
If I could only do a one day boat, I'd search for a boat chartered by a photography group as they're more likely to focus on gorgeous sites rather than needing safer spots for OW classes. (Or hunting spots if there are spearfishermen on board.)
As several have mentioned, I also prefer Anacapa / Santa Cruz for one day boats as you spend more time diving and less time motoring... but your freeway time is something to factor in.
Going to Avalon with a hotel stay in July / August can be $$$$.
Depends on your budget, of course.
Using the dive boats from Avalon adds to an already $$$$ trip. They'll dive many of the same places that boats from the mainland visit, but you'll be there a couple of hours earlier. The dive park is lovely. If this was my only trip to SoCal, I'd want to see more than just the dive park, especially as the weekend crowds in summer can detract from the dive experience. It's a charming town, but you'll be more and better diving from a boat.
Not all local boats are equal in service and spirit.
Please feel welcome to PM for a more confidential view on the best match your diving goals to the boat most likely to succeed.
(I'm not affiliated with any boat, just a frequent customer of several.)
Have fun!!
Gear up for cold water (50 is possible below 100fsw all year long) so you can enjoy all depths and conditions!
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Claudette