I know there are new ultrasonic speed sensors but I hadn’t heard of them being “through the hull.” I got as far as looking up the price, which is a significant fraction of the value of my boat, and put them out of mind.
I did use the depth sensor in “through the hull” mode for a year before drilling a hole for it at the first opportunity. The problem was that it would blank out in very shallow water - exactly when it was needed most. At the time I was docked up in the back end of a shallow swamp and needed to feel my way very carefully. A foot of water under the keel was a good day. I suspect there were too many reflections at close range. Anyway, the blanking problem never occurred again after installing in a hole.
I thought I had saved the core sample, but can’t find it - maybe I have a picture of it somewhere? I think it was well over an inch thick - forward and outboard of the keel a bit.