I have a springhouse sitting atop a mostly buried, 14' X 14' open-top
concrete cistern. The plywood flooring is rotted and needs to be
replaced. Though the cistern has some vents below the floor level,
the underside of the floor is essentially a high-humidity environment.
What is the best material or construction to use for this floor?
Things I have considered:

1) pressure-treated plywood (rejected, since this sits just over our
drinking water),

2) Marine-grade plywood (very expensive)

3) exterior-grade plywood, installed over a vapor barrier (6 mil poly sheeting?)

What will work best?

Thanks,

Kelly