ScubaWim:
We will be arriving the 17th and leave again on the 21st. What would be a better thing to do Catalina or the Channel Islands?
Catalina *is* one of the channel islands.
Cost is similar, once everything is added up.
Its really a personal decision, diving either place will be great.
If you take the ferry to catalina, you can dive off the shore at your own pace, and you can rent all the gear on the island. you still have to worry about carrying gear from the dive shop to the dive site, and back. Or you can take a taixi. Entry and exit is fairly easy on the island. The dive site is called "casino point" or simply "the dive park". If it is a friday-sunday, you can get air fills right at the dive site. You will do all your dives at one place.
If you take a boat to anacapa, you will do 3 or 4 dives off the boat. your gear will be waiting on the boat. food and air fills may or may not be included in the price.
either way you will have to drive 45min-1 hour from santa monica to catch the ferry or dive boat.
hope this helps! I'm sure others will have more info to add
By the way, there is an entire thread that talks about taking the ferry to catalina island to dive on your own, here it is:
http://www.scubaboard.com/showthread.php?t=85342
Scott