It just occurred to me that the boat is in his wife's name not his name but her signature is on the pink slip.
In California, possession of the "pink Slip" is a valuable tool. If the document has been appropriately signed off, you may be able to file the transfer with the DMV and obtain legal possession. Actual possession could be a different matter. The documents, note and pink slip, would need to be reviewed to provide a definitive response.
I would suggest that you use Avvo to locate a business/litigation attorney in your area and provide him/her with the documents to review.
The information presented here is general in nature and is not intended, to be construed as legal advice. This posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, consult your own attorney.
Debt Collection Attorney
If you have a security agreement which gives you the right to repossess in the event of default in payment, you may exercise such rights, take possession of the boat and auction it in accordance with the laws of the state where it is situated.