A lot depends on the boat, what it’s used for, it’s expected lifetime, and how long it stays in the water at any one time. For my purposes, future builds will use marine grade plywood (fir; nothing exotic) only for boats that I just leave in the water, and then only below the waterline. And maybe not even then. Depending on the boat, the price difference between marine and exterior grade might pay for a boat lift or rail system so that the boat never has extended periods in the water.