In my experience pigs will create their own wallow, and it usually involves mud, not just water, so the worry is you might spend loads of time and effort making a super wallow with stone sides and bottom and your pigs will ignore it and dig one elsewhere. In one of my pig areas i have a wonderful old pond (made by the previous owner) that must be thrity feet across and always has a couple of feet of water in it, and right beside it is a sticky gooey muddy wallow the pigs created and in the summer, that's where they spend their time, not the pond.