I live in N Cal and just bought a boat in Havasu, I am not sure of how much of this will apply to your particular situation but here are all the things i found during my research.
Luxury Tax- There is no luxury tax in Ca or any state for that matter, there is a 1% (of the value of the boat) property tax in Ca.
AZ Sales Tax- in AZ there is no sales tax between PRIVATE parties, however there is sales tax if purchased from a dealer. I believe the tax rate is 7.8% in havasu.
CA Sales Tax - if the boat is brought into CA and registered within 12 mo. of purchase you will be required to pay the tax of the county you register the boat in, however when you provide proof of sales tax already paid CA will get the tax from AZ and either credit you or charge you the difference in the two tax rates.
Hopefully this helps a little, also you can call the state board of equalization (CA or AZ) and they can help answer questions on this matter as well.