The two closest islands - Anacapa and Santa Cruz - are on Island Packers' December schedule for landing trips. The Channel Islands are essentially wilderness areas out in the middle of the ocean - no cars, no businesses, few buildings, few to no roads, primitive facilities, but with seals, sea lions, seagulls, guillemots, oystercatchers, shearwaters, dolphins, whales, an occasional shark, kelp forests, garibaldi, endemic plants and beautiful vistas. In season, the wildlife, wildflowers, swimming, hiking, kayaking, snorkeling, and camping makes the Channel Islands a world-class worthy outdoors destination.
Is it too cold? Well, it sprinkled rain this A.M. and was cold enuf to need the heater. Right now, it's dry and partly cloudy - very pleasant, but it is mostly sweater weather. When out on the boat and outdoors on the island, you will want to bring warm outdoor clothes. Shorts and t-shirts won't be warm enough. You can sit inside the cabin for the ride over.
Is it too rough? (I meant to look out on the ocean, but forgot.) Yeah, it can sometimes get pretty miserable out on the ocean, in any season. Things to consider... I think you can make a decision on whether to go the day prior. Check the forecast & ocean. Island Packers will leave a message in the A.M. for you to call if they cancel. Also, it only takes about an hour to get out to Anacapa or Santa Cruz - you have to decide on which island for a landing tour. So, before you get too nauseated and wish you were dead, you are getting off the boat onto the island or back to shore. Non-landing tours can be as long as three hours of misery if the seas are rocking.
Is all day too long to spend? Well, that depends on your interests. If you are able to schedule a kayak tour, (with a company like SBSuz's SB Adventure Company), you will have the adventure of your life. I've camped on nearly all the islands. I thoroughly enjoy the islands.
Hiking Anacapa Island takes about an hour and a half or more. During winter, the plants are greening up because of all the rain, but there won't be many flowers, or nesting birds. The view from the west end of the island looking towards the other two islets and SC Island is very pretty. There are no NPS guides to show you around - you don't really need a guide. There is a very small museum. There is a classic lighthouse, but you aren't allowed to get too close. There are picnic benches available. There are no beaches (except at Frenchy's Cove, which is only accessible by kayak or special trip by IP.)
Santa Cruz Island is the largest of the Channel Islands. You have a choice of landings - Prisoners Harbor or Scorpion Cove. I like the hike at Prisoners to Pelican Cove. Hikers here must be accompanied by a guide. You will walk through and around oak woodlands and arroyos and near the ironwood grove. Check with Island Packers to ensure that the hike will take place. This rugged but very doable hike takes about 2 - 3 hours, round trip.
Otherwise, Scorpion Cove hiking (Santa Cruz Island) is more desolate, and you are on your own. The cove itself is nice, but better to have a kayak - there are caves and pieces of a shipwreck to explore; and swimming is better during the summer. The hike over the hill to Smuggler's Cove is a bit boring and exhausting. I usually have a kayak, so I haven't tried this hike, but I think I would enjoy the hike to view Potato Harbor better than the hike to Smuggler's. There are some historic ranch buildings, (not really much to look at), to view at Scorpion Cove.