Peat moss tends to be quite messy, and apparently harbours asperilligus, however herpers have used it for decades with no problems, so draw what you wish from that. Cypress mulch is a nice substrate to use, you have to be careful that is does not lead to impaction however, if the snake feeds on it. Ground coconut is another fairly good one, as long as it doesn't dry out (then it gets dusty), however it is also quite messy. With the really fine ones (peat moss, ground coconut husk) you will have to be changing the water a lot more. If you can find bark nuggets that are not made from any softwoods or walnut, that would work quite nicely as well, and your snake is less likely to swallow one