Changing thru-hull valve in the water
Here is a method I just used that might be of interest (photos are in order of sequence):
I wanted to replace a poor quality 1-1/4" valve in the head with a proper bronze valve. The valve is on the hull bottom, a few inches below the water line and I looked for a safe way to do this without pulling the boat out of the water.
At first I looked at an inflatable packer (Cherne 270008 Rubber Test Ball Plug, 3/4"-1-1/4" on Amazon), then I thought about trying to make a mechanical expanding packer, then I discovered that you can buy these inexpensively - they are sold for testing plumbing. (C$17 - Cherne 269883 Kwik 'n Sure 1-1/4-Inch Plug on Amazon)
The test plug that I bought had a big wing nut on it and needed some modification to be able to insert it through the valve. I added threaded rod and a piece of metal tube (from my scraps box) and assembled the "packer" device shown in the photo.
I added a short piece of ABS pipe to the top of the old hull valve, such that the top of the ABS was above the water line. I then opened the valve and could see daylight through it and I cleaned the bore of the thru-hull skin fitting with a mini wire brush taped to a stick.
I then inserted my packer and tightened the wing nut to expand the rubber bung. I then unscrewed the old valve and slid it off the packer extension. I gave the extension of the packer a good "wiggle" to see how firm it was and found it was very solidly held. I cleaned the threads on the skin fitting, applied Teflon tape and installed the new valve.
Next I installed the ABS pipe again, loosened the wing nut, pulled out the packer and closed the new valve.
Before doing this in-the-water changeout I was somewhat nervous, but the operation went very smoothly and I would not hesitate to do it again. The longest part of the whole operation was cleaning the threads of the skin fitting prior to installing the new valve. I think that the inflatable packer would also work. It is a bit more expensive ($57) and you need a pump (bicycle pump?), but it would not need modification.