it will keep the plywood from warping and it is safe?
Polyurathane is used on floors and is very durable. it will seal the wood which should prevent warping although you don't want to let wood get soaked. The waterbased products are much lower in VOC's (votile organic compounds) and once completely dry (no more smell) should be water resistant and safe. Get a high quality one that can be used for floors. The water based may need 3-4 coats but it dries pretty quick.
Oil based polys have much stronger thinners and chemicals.