I've built ponds before, so I was already familiar with the process. I don't know how much water you'd lose from soil percolation; I've always just used a line

I've built ponds before, so I was already familiar with the process. I don't know how much water you'd lose from soil percolation; I've always just used a liner. EPDM rubber is what most people use. It's not terribly expensive and can be purchased in surprisingly large continuous sheets, or multiple sheets can be glued together if you want to go even bigger. I keep koi in my pond. Koi have particular requirements with respect to aeration and filtration and depth. If you're just building a general purpose pond those aren't as much of an issue. Be aware that if you use herbicides or pesticides that you don't want to end up in the pond, then you probably shouldn't design the pond to capture the maximum amount of runoff. You'll probably want to at least get some mosquito fish to keep mosquitoes under control. Mosquito dunks are another solution -- they're these floating things you toss in the pond that contain a bacteria that's specifically harmful to mosquito larvae. Some pond plants can survive a freeze (cattail, azolla, lily pads, horse tail) whereas others don't (water lettuce, water hyacinth, taro). I'll usually buy a clump or two of water lettuce and water hyacinth in the spring, and then by the end of summer I have so much I have to fish it out of the pond to keep it from taking over completely.