For this radar level sensor, it can be done in stainless steel or alumimum housing. to measure seawater, which material is better?

 

The material you mentioned is related to the housing of the transmitter. It is actually not in contact with the media, if the sensor is mounted on top of a tank or something.

 

As you mentioned it should be used for Sea water, SS housing to better for corrosion.
The metals most commonly used today in the construction of marine structures and fittings are stainless steel and aluminum. While aluminum is light, it has limited corrosion resistance in seawater and requires significant maintenance.