One of the best alternatives to marine plywood is fiber cement board. A fiber cement board is made out of a combination of cement and wood.
The material is very strong and it is not susceptible to water damage like some of the other types of plywood on the market.
Another option that you may want to consider is ribbon-grain plywood. This type of plywood has a very smooth surface and it is also extremely durable.
Both of these options are generally less expensive than marine-grade plywood and they can be used in a lot of the same applications.