One that I enjoyed was Valve Open on Bradford Island. You can pay a few bucks and take the ferry, like I did, or use your own boat. I'm sure the owner of the Travel Bug I left there last October would appreciate it being set free.
A few miles up the Sacramento River is The Tomb Of A Million Souls and The Black Scarab, although the latter may have been lost to the shifting sands of time. These are actually accessible by overland route, but a boat can get you much closer.
Much further upstream is Coyote Island Cache but I think the nearby slough is more appropriate for a dingy than a boat capable of getting there from Antioch.
Lil Devil, thanks for the tips. I'll try these next weekend...oops mother's day. Unless I can convince the wife the day on the water is for her I'll have to try in a few weeks.