The other posters are correct - Marin county, or any county in California for that matter, does not have any role in collecting sales/use tax. That is the job of the CA BOE, or if it is not USCG documented the CA DMV.
Sales/Use Tax - what you pay the State of CA (one time) when you acquire any
tangible personal property or physical property other than real estate, new or used, generally from 7.25-10.5% depending on the county in which you reside.
Property Tax - what you pay your County annually based on it's assessed value, generally 1-1.45% of assessed value.
In this manner all property eventually reverts to the state.
It is your responsibility to avoid, not avert, property and use taxes by every legal means possible. Same goes for all other taxes.
Both the Counties and the BOE (state) are absolutely ruthless in going after revenue, and not all counties act with the same policies/appeals process with regard to property tax. Bay area counties are notoriously aggressive. Generally, you have to follow their local appeal process which involves a review board, who's decision are swift and final, and you can expect in most cases to lose the appeal if the boat is berthed in California for any length of time. There is no recourse in the courts.
Over ten years ago, I purchased an expensive boat in Oregon and left it there to use as a second home. To make sure I did not get taxed by the CA BOE in arrears, I filed a BOE tax return on the boat once I changed plans and brought it into California over a year later. I was able to document that I used it regularly (airfare and fuel purchases in Oregon, a company local office in Oregon, etc.), I didn't have any intent of moving it to CA when I purchased it (no advance paperwork on relocation service, proof of long term slip rental, etc...).
In the end I did not get taxed on my purchase through the formal BOE process.
No county in CA should use any state's sales/use tax as part of the basis for valuation of a boat for property tax. You should easily appeal the valuation in my opinion. However, if you keep it in a municipal (city or county owned) marina you should anticipate that you will also be taxed on the value of land
under your slip where it is kept. In addition you will be charged for non-secured property tax in the county in which it was located on Jan 1 of each year typically going back 8-years. Appealing the valuation could trigger a review of prior taxes paid or skipped.
My county (San Mateo) charged me in arrears for the full 8-years of property tax on a trailered fishing boat that spent Jan 1 of each of the preceding years in other CA counties. I argued that they should coordinate with the other counties to determine if I did or did not pay property taxes in those years to the other counties and they argued it was
my obligation to prove I paid elsewhere. Fact is – I didn't pay for those years and so they had me over a barrel. I believe that the formal rule is that you are required to report the boats location to the county tax collector on each Jan 1st - no idea how you would do that.
Disclaimer - I am an expert at paying taxes, not avoiding them. Suggest you follow up with your local friends and tax experts here to see what your options are...
https://www.marincounty.org/depts/ar...assessor/boats
Here's the marin county vessel property tax statement you "are required to file annually"...but only if you are a documented commercial vessel, so I don't know what you do when they don't find and bill you.
http://www.capropeforms.org/forms/Marin/BOE-576-D/pdf/
Oh that reminds me - I forgot to file my report this year