I agree with the majority here, treated lumber is normally the cheapest you can get, it's soft and wicks water throughout, the price is in the treating.
Most modern treated wood uses copper azole, which has been pushed in many states as better healthwise than the old style, but still has the same materials as the same states are trying to ban from bottom paint.
With the copper based treatments you need triple coated screws and have to observe all the same disssimilar metals rules, Just as though you were bolting various metals together.
cabinet grade plywood is great for interior projects on a boat because it is normaly a nice surface on both sides, and while NOT cheap, still cheaper than marine grades.
-Normally- the need for void free plies is when you have water contact, it can hold water and delaminate, or when using it to build a hull or other curved surface the voids are weak spots and either cause uneven bending or potential breaks.
For soles I'd be happy with a good one face exterior ply, if one face is hidden, why pay for two good faces?