[Mullins]

Tone - it's for freediving, not spearfishing.

Freedivers need a descent line to follow, to which they are attached by a lanyard. There needs to be quite a lot of buoyancy at the top, both for safety and for doing free immersion. The depth sounder is useful because it lets you check the depth before you put the line down. If you're about to do a 70m CW dive, it's not very practical to swim down to 75m first just to make sure it's all clear.