Apparently, its some kind of secret handshake thing that older cruisers only tell the newbies after they have been out long enough, but it turns out you should just go to your Sam's Club or Publix, or Insert Big Bulk Store Here and get the largest, most obnoxiously sized package of the absolute cheapest junk on the shelf.
Invariably, this will cost 15 cents per roll and will be of such construction that actually calling it 1 ply sounds wildly optimistic. The shoddy construction of these rolls is what makes them agreeable for marine use. While our head (toilet) says specifically that you should only use "marine grade" paper in it, we have had zero problems with the cheap stuff in over 6 months and probably saved $60 over the course of that time.
Remember, we are sensitive to sewage.