I have a question, if I am at 50 m. deep, and my GP1871F says there is 24 ºC in the water, that is the surface temperature or the bottom temperature? I have a B45DT transducer.
The temp sensor is inside the transducer so any temp information comes from there unless you have a NMEA 2000 source you have selected to us. The temp would be considered surface temp because most transducers are not very deep into the water.