Contact paper is not really designed to be waterproof but it sort of is. Somewhere on the web somebody has a picture of a MDF box lined with contact paper that is full of water and evidently has held water for a long time.
I have used contact paper in a few applications and have generally been very impressed with its durability. Unfortunately it is now thinner and made with a weaker adhesive and I hear from some snake keepers that it's not very good. I have not used these newer contact papers.
For a sealer to put over melamine, there are not a lot of choices. Even if you sand melamine aggressively very few paints will stick to it. I have only found two products that will stick to melamine. One was something a paint store sold me as "flash primer" that I was using to prime glass that needed to be painted. I had some left over and tried it on melamine and it worked.
But they also make a product called melamine paint that is often used to paint melamine cabinets. I have used this product in reptile cages but I can't say that I really tested it long term in a variety of conditions. My impression was that it was a durable paint but I gave the cage away to a friend who was moving out of state and never followed up on how it held up.
I suspect that some of the plastic spray-can paints will stick to melamine but I don't know how much protection they will offer. The solvent-based epoxy paints will probably stick but with as expensive as they are I'm not sure I'd even try putting them on melamine.
To answer the questions from your other post...
Vinyl film is used in the graphics industry and is very durable stuff and actually cheaper when bought in bulk. If you buy just enough for a few cages at a sign shop it will almost certainly be more expensive than contact paper. I also don't like how glossy it is. It comes in several grades. For a reptile cage you really just need a basic calendared vinyl film rated for 3-5 years of use. The more expensive and longer rated films offer no additional durability in a reptile cage application.
Lastly, many people do use linoleum or some sort of vinyl flooring in reptile cages with good success. Works very well and can often be had for free.
I think that scrap vinyl flooring on the floor and first few inches of the cage walls and then melamine paint on the rest of the walls will offer sufficient protection for most reptile enclosures.
-----
Currently keeping a small collection of various Gonyosoma. Both G. janseni and G. oxycephala.