There are many types of ply and many methods for making plywood. Marine grade ply is a high quality ply without voids inside the layers of wood, and without repairs to the wood veneers inside the ply or the surface.

When you begin to stress the ply, any voids in the ply will cause problems (ply will probably break). You can repair the voids by filling them with epoxy, but it is very time consuming and generally not worth it. If you dont fill the voids, and the ply survive the stressing/steaming, you can get problems later with strength, moisture and probably delamination as well.
Marine plywood is usually made from mahogany, which is a rot-resistant high density and strong wood. Gaboon okume is the most usual one.
Regular plywood are often made from woods with properties less good than Gaboon okume (strenght/weight). Often there is just one good surface, and a core made from softer low-quality wood. Lately, there has come some mahogany skinned plywood with poplar as core on the marked for use in furniture. Avoid it for use in boats!

Plantation grown okume grows too fast, so it is not as good as the plywood made earlier from forests. But we got to think about the environment..

If you think about it, the plywood is a relatively minor investment compared to what other funds you will put into the boat when you buy sails or deck gear.

In short. You can use other ply, but there are severe risks.
Perhaps Phill will have some good ideas for either sourcing some marine ply or other alternatives.

A last tought. If you could get okume veneers, it would be relatively easy and straight forward to make your own high quality plywood..