There is a lot of talk about what product to use but I think you guys are missing a real important point.
NO WOODEN PRODUCT WILL WORK WELL UNDER WATER!!!!
All of these products have their qualities and the problems. If you understand a little about that and use the product correctly, you will be sucessful with any one of them.
For the people below who say "melamine is crap" I might suggest that you used it in some way it was not intended to be used or treated it improperly.
I own a cabinet shop and use approx 1000 sheets of melamine each month. It is the standard of the kitchen industry and any quality cabinet will use it for the interior of the cabinet.
Fused Melamine (different than cheap cold rolled melamine) is the same plastic that formica is made of. It is highly abrasion resistant and nearly water proof. That is the surface of the board. The core of the board is particle board. This is simply sawdust compressed and glued together. The quality of the board depends on the compression. Industrial grade or 45 pound board is the best grade, construction grade is less.
MDF stands for medium density fiberboard. It is different from partical board in that it is not sawdust but rather wood fibers compressed and glued together. Neither MDF or particle board is water proof in any way!! If you use either of these products you need to treat it in some way to protect the board for water or even high humidity.
With melamine, the surface of the board is all ready protected from moisture. MDF will need to have the surface treated. Both need to have the edges coated with paint, varnish, silicone, or some water proofing agent.
MDF's main use in the cabinet industry is for paint grade cabinets and furniture. It is also used as a substraight under various hardwood veneers. It's main quality is that the surface is very smooth. Good for painting.
Both particle board and MDF are very stable and resist warping.
Plywood is generally not smooth on the surface. It is not water proof in any way, it needs to be painted or sealed. It warps like a bugger no matter how you treat it and for that reason is rarely used in cabinets any more. It is much lighter and easer to handle than mdf or pt. bd.
All three of these products need to be glued when assembling. When using screws, they need to be predrilled to prevent splittling. None of these products hold nails very well. Nails go in easy and look good at first but loosen with time in all three products.
If you have to tools ability to work with it, Melamine will be the best material for most snake cages and racks. I have melamine cages that are 20 years old and show no sign of wear.

