I'm assuming also you're a California resident. How many months a year do you expect to use the boat in each state?
You're dealing with a rather complex situation if you intend to use it nearly equally or even several months per year in each state.
It starts with sales tax and where you purchase the boat. You can get exemption from that state's but then you're responsible for the other. However, if you purchase it in one state, decide to take it and register it in the other, there are some limitations on returning with it to the unpaid state. As an example if you purchased in Washington, took it to CA and registered there, it can require a use permit for WA and once that is used may limit you returning to WA for a period of time. And the rules are different based on size.
California's issue is similar. Let's say you purchase in CA and intend to take it to WA and register there. If you stop and make any recreational use of the boat in CA then it's subject to use tax.
In WA most boats are subject to an excise tax whereas it's a property tax in CA. The excise tax is generally half or less of the property tax. Now it was mentioned not having it in CA on January 1. This does apply for a sale. If you purchased it and brought it in on Jan 2 then you wouldn't pay until the following year. However, if you're a resident in CA and keep it in CA a substantial part of the time, CA goes by habitual place of mooring. That is if the boat is normally or often moored in CA and you live in CA then it's going to be subject to property tax in CA regardless of where it is on Jan 1. That only changes when it becomes permanently situated in another state.
If it's principally used in CA it must be registered there. WA would then not require registration for a non resident if you got permits as described above which allow its use in WA up to six months of any consecutive 12 months. Note it's not of a year. But if you used it five months, then one month three months later, you couldn't return without registering for until 12 months passed starting with the first month you used it there. Then you could go another five, but not six, until it had been a year since that single month a year ago.
It's all terribly complex and I think the two key questions are where do you reside and where will the boat be a majority of the time. But if you reside in CA and the boat is docked in CA a significant amount of time even if under 6 months a year and even if not on Jan 1, CA is going to expect you to pay property taxes and it would be very difficult for you to argue against them. If you only brought it to CA one month a year, perhaps, but not with split use.
Regardless one or the other state is going to get you in all areas. And if you try to get two creative both will come after use tax or property/excise taxes or say you need to register it.
Don't start down the path of trying to create the scenario with the least taxes. Any action taken strictly to try to avoid a tax can easily backfire. Think about what you really want to do. If that is you live in CA and want to use it there most of the time, then pay sales tax or use tax there, register it there, and pay property taxes there. Then look carefully at the regulations of other states if you purchase it in one and/or you intend to spend a lot of time in one.
Good luck with enjoying the entire west coast and any other areas you cruise to.
