I love these discussions of transom plugs. They are full of great reasons for each position always, and ultimately I am not sure it often matters which the outcome is.
I count on Larry to be right about most things and rarely disagree with him. In fact I don't really disagree with him on this, as his reasoning is solid. I just disagree as far as my own practice goes.
I used to put my plug on the inside, but for a long time have put in on the outside.
Why? It's way easier than climbing into the boat and hunching down to get to the inside of the transom. That DEFINITELY is not reason itself, but it tips the scales, all things being equal.
I absolutely agree with Larry about the screw type plug. Use those only. Collect 'em. Have more than you can ever need, and have them handy.
An "inny" argument is that a vandal can tamper with the plug from the outside, removing it and having you sink. If that is an issue, put the plug inside. If not, outside works just fine. If you somehow don't tighten it adequately, water pressure is in your favor, helping to hold the plug in.
And if somehow in the unlikely possibility you lose the plug, by all means install one from the inside, at any time!
Meanwhile, spend your extra time, the time you saved not climbing in and out, to spend a few extra moments on the water.
And Larry, by the way, just how do you know it takes 15 minutes for the cockpit to fill. There's got to be a story there!
Dave