Good question
Marine ply has carcinogens in the ply used during the process of pressure treating the wood. The marine ply also breaks down faster then one would think when in a wet environment. Marine ply is also heavy, and we go back to the carcinogens when working with the ply. There are several grades of outdoor ply's that use water resistant glues and are coated to resist moisture. These ply's can be skined/veneered, laminated, or finished and will outlast any conventional construction you see on the market now by many years. Basically you have a forever cabinet. There is a price for these types of ply but not much more the good cabinet grade ply's
Wood cabinet ply is a good choice but need to have the edges pre finished so that the joinery if wet will not such the moisture in like a sponge, and proper finish used on the outside. Another thing to look for is a 11 ply cabinet grade ply and not the 5 layer ply's. Most improvement stores will have cabinet grade ply but not of a quality that is bought wholesale by shops building, some wholesale supply places will sell to the public. Even in the wholesale purchasing market one has to fearful of what is being sold, source a good source. Chinese ply has so much formaldehyde in the glue one can smell it and one needs to stay away from that too, remember the Chinese dry, same thing.
Building out of hardwood is another option and using the proper finish.
Another recommendation would be is to have a top larger then the cabinet in case of spills, so that the water does not go inside of the cabinets but around the outside or on the floor.
Most of the manufactured cabinets that come with the tanks are a melamine covered cheap substrate and once scratched, chipped or moisture stand on a edge or joint the substrate is compromised. The manufacture is going to use the most cost effective way to product there product.
I hope this helps an answers your question.
