Correct. The sonar ping is sent to the bottom, wherever that may be, reflects off the ocean floor, and bounces back. The first thing it will hit on it's way up is the keel. The time it takes for it to return is how your nav officer calculates the depth.
It's actually more useful than being told how deep the ocean is; say you want to dive to avoid a hunter-killer patrol in shallow waters. If you're already at periscope depth (anywhere from 12 to 18 metres), and you ask your NO what depth under keel is, then you know exactly how much room you have to manouver before planting the bow into the ocean floor. If you're only given "Sir, the ocean floor is 30 metres deep here sir!", then you'll have to do some quick math, wasting time in a situation where every second counts.
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"That flag and I are twins, born in the same hour from the same womb of destiny. We cannot be parted in life or in death; so long as we float, we shall float together."
As much as I dislike it sometimes, I'm a tin can sailor, through and through.
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