How can ships use ultrasound to detect the depth of water beneath them?

Submarines use standard fathometers, though they're more powerful and more focused than most commercially available units, for depth measurement. The actual frequency used is classified, but it's in the upper Kilohertz range and short pulsed to minimize detection when used during submerged operations close to shore. Anything above 20khz is considered ultrasound.

In the deep ocean they're typically not used, as most boats rely on navigational charts for depth estimates. Most areas of the oceans are deeper than design limits of submarines anyway, so they're rarely used in deep water unless you need to verify a chart depth.