The Channel Islands are five outstanding islands. Great kayaking and snorkeling are to be had here. I've gone twice, both times in Summer, so that's all I can speak to.
The five islands are accessible by plane year-round though by boat only Anacapa Island and Santa Cruz Island. However, accessibility is always weather depending. The other three islands San Miguel, Santa Barbara, and Santa Rosa are accessible by boat from April to October/November. Boat trips are not offered daily. Booking in advance is essential. Check the conditions and closures of the islands first. Channel Islands Conditions
Most visitors come during the summer from June to August. Due to the limited transportation opportunities, it doesn’t get too crowded.
The islands have a cool Mediterranean climate, but weather varies considerably from island to island. The winter is cool and wet - summer is warm and often dry. Most precipitation from November to April with its peak in January and February and drier from May to October. Summer is the driest period. The climate is influenced a lot by the sea. Fog may appear, especially on the outer islands during the night but can occur at any time. The islands are often windy, and strong winds occur on the outer islands. Strong eastern winds from October to January. Sea temperature is around 50°F (10°C) in winter and around 65°F (18°C) at the end of summer.
Each season has its own character, different weather, and outdoor activities. The best time to visit one of the islands depends on what you are looking for as well as the activities you’re planning:
Snorkeling: It's snorkelers paradise and best at the end of summer until fall with a sea temperature of about 65°F (18°C). A wetsuit is recommend to stay longer in the water.
Watersports: Besides snorkeling, summer until fall is also the best time for sailing diving, kayaking, and swimming. Many consider early fall as the best time of year for diving or snorkeling as ocean temperatures go up to 70° (F) and visibility may reach 100 feet.
Whale Watching: January until March has the highest chance for gray whale sightings.
April to September is the season for blue and humpback whales due to the high plankton concentration. Year-round spottings: orcas, minke whales and different species of dolphins. Besides, it is common to spot seals, sea lions and water birds.
Birding: Birding is outstanding in the Channel Islands. Some endemic birds occur here like the Island Scrub Jay on Santa Cruz Island. Best time is the nesting season in spring. Shorebirds are best seen on Santa Cruz, Santa Rosa, and San Miguel.
Surfing: In the winter and spring the north shore is best, and in the summer and fall it’s the south shore. Best access to all surf spots by private boat due to the remote locations. Besides being one of the best times for surfing, in the winter also some of the best sunsets occur.
Wildflowers: Depending on the amount of rain from the end of winter until spring some continue to bloom during summer. Gumplant, buckwheat, poppies, and verbena can still be viewed in the summer.
I've backpacked in the Kings Canyon area twice, in May and September, which are what I would consider the bookends of the season. Any colder, and this would be for serious Winter lovers only.
May through October