Is there a way to measure the angle of the seabed? The reason for the question is that this is a scanning of a rockfilling where there will be build a house and the constructor would like to know the angle of this filling.

There is no function within ReefMaster for this. All I can really suggest is that you use the measuring tool the measure the horizontal distance of the slope, then combine with the depth distance and use trigonometry to calculate the angle of the slope. There are quite a few online triangle calculators that can help with this part.