In response to removing the plug completely, this can be done on the trailer. The center running board on the trailer (under the centerboard) should not be supporting any significant weight. If it is, you can run a ratchet strap around the entire boat to pull the board up off the trailer running board. You can then just move the running board and move it out the way to one side. I did this by hand very easily. There are 3-4 nuts holding this to the trailer that you must take out. (good time to replace the carpet on the trailer running board).
If you are working alone, It's important to remove the plug completely and let it drop down to the running board before you move the board to one side. Otherwise, you can have someone in the cabin removing the nut while someone below holds the plug as it lowers. After replacing the gasket which was just rubber gasket material I purchased at the hardware store It was not easy to put back in, took maybe 3-4 minutes to find the hole inside the cabin, definitely a "suck it and see" type of deal. I added some anti seize to the threads to allow for smooth tightening prior to inserting the threaded bar. I could not imagine attempting to do this in the water, even with scuba gear it would be extremely difficult.