I have used just water for Pacs before but not as a permanent arrangement. I kept babies and juveniles this way because it was the easiest method, but that was also ten years ago and these days I know better. Pacman frogs dont live in shallow water, they like to burrow. Modelling the terrarium enviroment to reflect their natural lifestyle and encourage natural behavior is best. Today I would at least use moistened paper towel, they can kick it around, dig in it, keep moist and feel hidden all in a little bit of paper towel.
Water also dirties up real fast. I know of an African bullfrog that was kept in shallow water, and the owner neglected to change the water for a week and the frog died from the toxicity.
Despite concerns with swallowing moss, I keep all of my Pacman Frogs and Fantasy Frog on moss. It retains moisture just fine and since I feed my Frogs by hand on a smooth plastic surface outside of the enclosure, never had a problem. Unless they decide to start eating the moss when Im not around (something I wouldnt put past a Pyxie or Budgetts Frog) Im not concerned about impaction risks. But again this is only because I feed them outside of the enclosure.
I dislike the coco fiber as a substrate because it leaves the frogs looking so dirty all of the time. If I were going to use a soil substrate I would probably use an organic topsoil mixed with a bit of sand. Of course, if I were feeding them in the cage, I wouldnt use the sand. Is there an impaction risk with natural soil?