Yeah.......with this new information, I won't even be attempting this until it's good and hot outside.
I have minimal instrumentation and I'm something of a restoral nut so I'd like to have it working.
I think what I'll do is dive on the thru-hull, and lightly insert a shallow, rubber plug with a string attached to it. That way if I encounter any problems removing the red plug (like damaging the interior fitting or something) the exterior plug will already be in place, preventing or slowing water from getting in.
If I encounter no problems, then I can just gently pop the rubber plug out with the egg beater and recover it with the string.
Anyway, that's all months into the future. Thanks for the clarification.