The altimeter data from your instrument is processed in different ways, either with the AST (Acoustic Surface Tracking) or the LE (Leading Edge) method. Which method is used depends on the application. 

The AST method finds the distance to the water-air boundary by detecting the largest value in the return signal, the so-called max peak (see figure). This method is used to detect the surface when measuring waves. 

The LE method detects the boundary by finding where the return echo has the largest change in value (see figure). This method is better suited for echoes that increase at the boundary but do not decrease rapidly. This is typical for water-ice boundaries, and the LE method is therefore mostly used to detect ice. 

Figure: a typical return signal with max peak (used in AST) and leading edge (LE) marked.