My favorite day boats for the Channel Islands are:
Ventura:
The Peace (offers nitrox, but the dives are often shallow so it's less important)
The Spectre (air)
These boats usually visit Anacapa Isl and/or Santa Cruz Island. Travel time is short (1.5 hours) so you usually get 4 dives in. Both have a hot tub. Peace offers all around great food. Spectre offers a fantastic BBQ Tri-tip/chicken lunch. Both boats have a full bunkroom and enclosed galley space.
San Pedro:
The Magician (nitrox, full galley/bunks)
The Sea Bass (only 12 divers. Small enclosed cabin. Sandwiches or burgers for lunch. Nitrox. Fantastic Oil Rig dives if conditions allow.)
Catalina Island is the closest to these boats (2-2.5 hours). Santa Barbara and San Clemente Islands are 5-6 hours away. Looong day.
Long Beach: (These boats have air only. Catalina is the closest island)
Great Escape
Sundiver
Bottomscratcher (cramped deck space, but they do have a Boston Whaler chase boat that can pick up and drop off divers around a site.)
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So, these boats would be my first choice for day diving the Channel Islands, and I actually prefer the diving at Anacapa Island and Santa Cruz. (Peace and Spectre) Parts of Anacapa have been completely protected for over 25 years and it thick and rich with life. The unprotected areas may be urchin barrens. Big difference when you're a photographer.
If you're in a wetsuit, know that our water temps stay in the 50's and low 60's until late June. July offers warming waters, and August/September/October will have high 60's and even 70 at the surface.
When the water is cold, boat diving in a wet suit is better if the boat has a fully enclosed galley available.
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Claudette