A thermometer designed for use in measuring the temperature of seawater. One form of this instrument consists of a mercury-in-glass thermometer protected by a perforated metal case. This is used to measure the temperature of a sample of seawater. Another form consists of a mercury-in-glass thermometer surrounded by a metal case that forms a well around the bulb of the thermometer. When the thermometer is raised from the water, a sample is retained in the well for temperature measurement. See reversing thermometer, bucket thermometer.