Elkhorn Slough is the largest tidal salt water marsh in California outside of the San Francisco Bay, and is on the migration route for many birds. The slough contains much wildlife - on a paddle you will likely see sea otters, harbor seals, and many types of birds. Also possible to see Sea Lions at the mouth of the slough.
On the north side of the slough, after the bend with the seal haul out, is a channel which will take you meandering through the marsh. This channel is navigable in most tide levels.
The slough is impacted by tides, so be prepared for currents. And the afternoon wind is generally off the ocean and blowing inland, making a return trip to the launch that much more challenging. If the tides are not working for a launch at the bay and paddling inland, you can always launch from Kirby Park (shown on map) and paddle towards the bay.
It is very important that you follow the rules related to ocean mammals and birds. Make sure you don't approach mammals, particularly when they are pulled out on shore.
Launching is free at the end of Jetty Road. Launch facilities are also available behind Monterey Bay Kayaks, but there is a daily parking charge.
Monterey Bay Kayaks and Kayak Connection are the two local outfitters. Both rent boats and lead tours into the slough.
From Santa Cruz, CA drive south on Highway 1 through Watsonville. The road will cross over the slough on a bridge at the power station. There is a launch ramp behind Monterey Bay Kayaks or beach launching available off of Jetty Road just north of there.
Monterey Bay Kayaks has a map online at http://montereybaykayaks.com/info/moss_paddle_map.php