Dear Mr. Dailey,
Thank you for posting your request on History Hub!
We suggest that you request a copy of his Official Military Personnel File (OMPF). OMPFs and medical records of enlisted men and officers of the U.S. Navy who were separated from the service after 1958 and before January 31, 1994 are located at NARA's National Personnel Records Center (NPRC), (Military Personnel Records), 1 Archives Drive, St. Louis, MO 63138-1002. To request these records, please mail a completed GSA Standard Form 180 to NPRC. Veterans and their next of kin also may use eVetRecs to request records. See eVetRecs Help for instructions. Certain information in the records is not available to the general public without the written consent of the veteran or his next of kin. For more information see Request Military Service Records.
Due to the COVID-19 pandemic, NPRC will continue servicing requests ONLY associated with medical treatments, burials, homeless veterans seeking admittance to a homeless shelter, and those involving the VA Home Loan program. If your request is urgent, please see Emergency Requests and Deadlines. Please refrain from submitting non-emergency requests such as replacement medals, administrative corrections, or records research until NPRC returns to pre-COVID staffing levels. Please check archives.gov/veterans for updates to the NPRC operating hours and status. We apologize for this inconvenience and appreciate your understanding and patience.
If you are able to share more information about which unit your brother-in-law served in or where he was stationed, then we may be able to identify additional relevant records for your search.
For an idea of what kinds of records are available in the National Archives Catalog relating to the Underwater Demolition Team (UDT) in the 1960s, including photographs and moving images, please see the following links:
Photograph of Underwater Demolition Team Eleven Decontaminating the Exterior of the Apollo 11 Command Module
We also located a non-profit site that may be helpful to your search: https://www.udtseal.org/
We hope this is helpful. Best of luck with your family research!