I am very adamant about this. Depth is ALWAYS the depth of the water, and NEVER the depth below the keel. I don't know how that Idea started, but it is a very bad one. The instrument has an offset adjustment specifically so that you can set the depth to the depth of water, use it.
Consider, you find a nice place to anchor, in about 10 feet of water, which reads 5' below the keel. You put out 3:1 scope, which is recommended for a modern anchor in good conditions, that would be 15' based on your depth sounder. But that measurement is really to your bow roller! Depth in this case is 17', not 5 feet! You just put out 15' of scope in 17' of depth! (depth of water plus distance to bow roller).
So, to anchor, you need to add the depth from the depth sounder, add the depth of the keel, and then add 2 feet, and then multiply by your intended scope. What a PITA, and at a time that is stressful for many at times. Most certainly, setting the depth to the depth under the keel doesn't make any math easier, and doesn't allow you to forget the depth of the keel. All is does is make your boat different from what that instrument is supposed to read and makes it confusing for anyone that has ever piloted any other boat.
Do I think this would actually happen while anchoring? Probably not. But it is far more ridiculous to think knowing your draft is 5', that you might not know that the depth sounder should always read more than 5 feet. If you can't figure that out, then 100%, you are going to make the former mistake while anchoring.
Want another reason? Navionics updates charts based on depth readings from users, automatically in some cases, from chart plotters or phone apps using Navionics charts. What a mess if some of those people use depth under keel without thinking about the consequences.
And if for no other reason, because practically every other boat reads depth of water, and because that is what the instrument is supposed to read, don't confuse experienced crew by being different.
The sounder will not work out of the water. Which is unfortunate because you can't change it while in the water.
The race car thing sounds fun. What is going on?