I have not been extremely impressed with water-based polyurethane. It seems very sensitive to water spills, humidity, and feces/urates (a huge concern with a large python). I do have it in my bearded dragon cage but that's about all I would use it for.
I typically have recommended oil-based products for such species as yours but I'm starting to re-think my position. I've done some looking into the high-glass acrylic paints and think they are much more durable and almost as easy to use.
MarkG on this forum recommended a high-gloss acrylic for the floor and first few inches of the cage walls. He also had the idea of putting Bondo body-fill onto the lower edges rather than silicone and then painting over that. I think it's a great idea.
The rest of the walls could then be sealed with whatever one preferred from a looks standpoint. I think water-based poly would be fine for the walls of a rock python cage.
Both products are so easy to work with that yearly touchups are easy.
I don't think you absolutely need a UTH for your rock python if you already have another heat source. Is that what you meant?