The trouble with waterproofing the plywood is that the water will seep through the concrete and just sit there on the subfloor with nowhere to go. This will cause odor and mold issues within a couple of years. So what you want to do is waterproof the surface of the concrete. Redgard will work for that, but it's somewhat fragile and needs to be covered with tile, as the wood might rub through it.
If this sauna will see only occasional use, and is a dry sauna, then you can probably do without water proofing. It will dry out between uses. Just "burn" the surface with a steel trowel during finishing and then paint it with a clear sealer.
If this is a heavy use wet sauna, then you want to treat it like a shower pan and build accordingly.
BTW, you want to put a sheet of poly or tarpaper on the plywood, and then staple galvanized lath to that and then place your concrete. The poly will keep the moisture in the concrete longer and allow it to cure strong, but is not a waterproofing.
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