I have worn center seam woodland mocs a lot when portraying an eastern longhunter. I have worn the mocs barefooted, with thick wool socks and made a second moc liner from blanket wool. Depending on my feet condition at the time and the general terrain I have cut a "Dr. Shoals" insert from what is called oil tanned or belt tanned weight leather. These are then slid into the moc as a buffer.
I never did it, but wanted to make an non-tanned deer hide, hair on (to the inside) winter moc. You could then wear your summer weight moc as a sock and slide it into the deer hair moc. It would be for very cold weather.
I will probably miss quote this, but "moccasins are just a decent way of going barefoot". Do not "EVER" think you can add oil, bear grease, lard or silicone (etc)and make a moccasin waterproof. You may get a commercially tanned leather to be water resistant for a few minutes. Also if you do not put any kind of "waterproof treatment" on the moc it will dry much faster.
Last, it is my thought that those who wore mocs as a way of life had soles like leather. The moc was to prevent briers and sticky things from getting between the toes as you walked or ran through the brush and brambles.
Here is a link to the eastern center seam moc...
http://www.nativetech.org/clothing/moccasin/mocinstr.html