Your ship being decommissioned wouldn't have a direct impact on any rating you would receive from the VA. However, the determination letter you received should have spelled out the reasons for the denial - no substantiating service records, lack of medical records, etc.

Were you working with a Veterans Service Organization* (DAV, VFW, etc.) or your state's office of Veterans Affairs*? They would be in the best situation to give you one-on-one advice about what would be needed for a specific VA disability claim to be processed favorably.

If you aren't, then I HIGHLY recommend that you reach out to a VSO or your state's veteran office/department and have a discussion.
---------------------------------------------
* VA recognized VSOs - https://www.va.gov/ogc/apps/accreditation/index.asp
* Links to each state's veteran office/department - https://department.va.gov/about/state-departments-of-veterans-affairs-office-locations/

LTC Kevin B. F.Y.I. ...