The "Main Axis" of the left and right SI beams are at about 30° from horizontal ...(The "Main Axis" is the strongest part of the sound pulse) ...
The sound pulse radiates outward (along the Main Axis and away from the Main Axis) in an ever decreasing sound strength ...
This allows the 455kHz sound pulse to cover from the water's surface (on both sides of the xducer) all the way down and overlapping under the boat ...before the sound pulse diminishes to a strength that wouldn't pick up echoes ...
This is the SI beam form of the right SI pulse ... Add a matching left SI pulse and it's evident that the SI covers a full 180° under the boat ...
I'm not understanding your question of depth coverage ...
The HB specs for the SI depth is 150ft ... This could be more or less dependent on bottom composition and water borne particulate ...
Rickie