There are two different types of tax to worry about. The largest is effectively a sales tax (called use tax, but same rate as sales tax -- varies a little by county, but close to 9% in most places, and applies to the entire purchase price (or value) of the boat. If you bought the boat out of state and kept and used it out of state for the entirety of the first year, even if you are a California resident, you will be free of the use (sales) tax. That is less onerous than states like Florida and Washington that will impose a sales/use tax, even on a boat I have owned for years, unless and only to the extent I can show I paid sales (use) tax elsewhere.
The other tax is property tax. California law is very complicated on that, but if your boat is here most of the year and/or in early January, you will get the property tax (about 1.2% of value).