You don't state which "Navy"...UK, US, Russian?
Individual Military service records are not Public Records, you have to request copies from the National Archives of the Country he served. Other gave you UK, this is US....http://www.archives.gov/veterans/
Now the U.S Navy only started "service numbers" in 1920 after WWI.
Having the service number helps with identify the correct records should there be one than one person with the name you are requesting.
Also all genealogy sites use names to search, not numbers, again the service number, if on a military record with name can help in identifying the found result as your ancestor.