Refractometers are the most accurate way to test your saltwater aquariums salinity. Specific Gravity is a very vital part of maintaining stability and having your inhabitants healthy. Make sure to try and maintain your parameters constant with salinity, one of the best ways to do this is with an Auotmatic Top Off system like the Tunze Osmolator or Smart ATO's.
Even though hydrometers will give you a somewhat accurate reading they are never 100% accurate. That is why we recommend always using a Refractometer when testing your aquarium salt levels (Specific Gravity).