Regarding time limits in WA, I thought it was a total of 6 months in any one year but in two 3 month blocks. Time between them can be as short as one day. But note that a 3 month block can be extended if the boat is having repairs being done. You just go down to the local licensing office and fill in the form, it is very straightforward.
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At 60 days you must obtain a non-resident vessel permit. Now, it gets more complicated because exemptions in registration are based on a boat registered in another state. Only boats legally registered elsewhere can get the non-resident permit. Sales tax will exempt or credit if you've paid elsewhere, but you can't get by with having paid none anywhere.
When we purchased in Washington, we registered at the time of purchase in Florida and paid Florida sales tax. Then we made sure we didn't consecutively stay in Washington over 60 days, by trips to Oregon, BC, and Alaska.
Now, add to all that the fact he's importing the boat. If he's going to keep it in California, he may be best off paying California sales tax immediately and registering there. Or, he could pay sales tax elsewhere and delay California entry.
Just get expert advice. I'd suggest a documentation company, even if not documenting.
Now, one other factor in his rush to move the boat in December. If it's in California on December 31, it will be subject to Property Tax for the year. Later entry would delay that a year.