What size "pool" are you looking for? (diameter across) Something large enough for a snake to actually swim in would be pretty hard to fit into a 6' enclosure- that sounds more like a room addition? I would also not recommend using the cheap plastic "kiddie pools" sold everywhere for humans this time of year- for one thing, the plastic could leach chemicals into the water that a snake would both drink & soak in (it wouldn't be HDPE type plastic), also they just don't hold up very well.
For large soaking-type water bowls, some use cat litter-boxes or the large cement mixing tubs sold in home improvement stores (again, not sure what might leach into the water from those, as no one drinks cement so it's not an issue when used for intended purpose). I would check out farm supply-feed stores for stock tanks & bowls used for large animals? Without knowing your intended dimensions, it's hard to answer.The mess that snakes make from swimming would seem to preclude putting a pool inside most enclosures.
For the sort of paludarium you seem to be talking about- you do need more height- I'm not a fan of enclosures with low ceilings anyway, they're a hassle to reach way in to clean, & not much room for "cage furniture" -things for climbing. I think there are materials you can use to make something- it's been quite a while but I vaguely recall someone using a product made for sealing the bottom of an outdoor pond that hardens up & is safe for fish etc. You should look into pond building products (waterfalls, etc).