Use form REG 135 Bill Of Sale
if you do not have, or did not sign the back of, the title. An alternative is to use form REG 138 Notice Of Transfer And Release Of Liability. On REG 138 leave odometer reading blank and fill in vessel HIN only. Each form is available as online fillable which you then print out. Remember to keep a copy for yourself...with date.
You may consider completing and submitting both forms and let the DMV figure it out. Until transfer is complete (recognized by state and county) you are on the hook for use tax and property tax. God help you if you are in Los Angeles County.
DMV.CA.GOV or DMV.ORG