Of all the things I've done on a boat the one I don't wish to do again is replace a thru-hull transducer while the boat is in the water. I did that once when we were docked in Ft. Lauderdale.
My dad elected me to be the one to put on the scuba gear and go down with the new transducer. We had already cut the wiring to the old one and removed the locknut. All that was needed was to knock the old one out and fish the wire from the new one through until the new transducer was seated. No problem!
I got into the water. It was murky with visibility only a few feet. I know it's silly but I thought of sharks sneaking up on me.
They didn't. I knocked on the hull, the sign to tell my dad I was ready. Then he started pounding on the old transducer with a rubber mallet. I thought my eardrums were going to burst!
I held my ears with my hands, no longer ready to feed the wire through. It seemed forever before the old transducer popped out.
Then I heard a muffled sound. My dad was panicking that the water was rushing in and there was no sign of me or the wire. I fumbled around and found the end and he grabbed it. He was pulling so fast I was doing everything I could to make sure it didn't knot. Finally the new transducer was seated and the flooding stopped.
When I got out of the water my head was still pounding. Of course my dad had no idea he almost blew my eardrums out.