May NOT be true!! some Macgregors even of the same model have two inspection plates inside the cabin to access the centerboard pivot bolt. In these cases, it is NOT accessible from outside. If you I fiberglass over the inspection port you may not be able to get to the bolt fort inspection or removal of the centerboard. I know for a fact the 26S was configured differently on some models. I have one that cannot be accessed from below the boat.
Research and inspect first!
The picture Pam posted is exactly the same in appearance to mine.
Couple of things here... Pam, your profile says you have a 1993 26X. In 93, it should be a 26S, not X. They stopped making the S in 95, when they started making the X. The 93 26S definitely has two inspection ports, just as you found, one under the sink, and one in front of the head. This is how you access the swing-keel pivot bolts on a 92-ish and later 26S. (That's if you need to drop the swing keel out from under the boat to fix it.)
The older S boats had access from two ports on the outside. Also, the 26X support for their swing keel is from the inside/outside, and different than what you found. (Has a hanger bolt and yoke.)
I'm going to assume you have an S not an X, because of what your picture shows. You can rest easy, nobody modified your boat incorrectly. It is as the factory built it (minus the leak, we hope).
Those plates open up directly to the big ballast tank, like the top on a thermos jug...wide open hole...on either side of the keel trunk (the space the keel hides in when you pull it up). So if the goop was un-gooping, and you had a leak, it was just from the ballast tank, and not from the outside in, another thing to be relieved about.
Hope this helps...