As has been noted Marine grade is best but also most expensive. Just so you know marine grade has nothing to do with rot resistance so make sure you seal with epoxy. Marine grade means there are less voids in between layers, it is better wood and if you can afford it by all means get it but it is not absolutely necessary. Sealing with the epoxy is very important, better to have good plywood and sealant than marine and no sealant. Just so you know.

Just make sure you never get pressure treated wood with an aluminum boat. Due to the process of pressure treating( with copper solution) there will be a chemical reaction- galvanic corrosion- and your aluminum will literally disintegrate.