Coralife - Hydrometer
Coralife hydrometers are easy to read and accurate to +/-0.001 specific gravity. It has an intake that is designed to prevent bubbles from making their way into the hydrometer potentially giving false readings.
Note: Before using the hydrometer for the first time make sure to fill the it with saltwater and let it soak for 24 hours.
Rivers to Reef says: Hydrometers are a great way to quickly measure the salinity of your tank. Realistically though, if you are keeping a sophisticated reef tank, such as an SPS dominant tank, we would recommend getting a refractometer. If you are doing a FOWLR tank or a tank with simple LPS and softies a refractometer will be great.