Teak and Holly ply comes in 1/4, 3/8 ( in some places ) 1/2 and 3/4.......
It is like any other ply such as teak, afromosia, mahogany: there is a very thin veneer of the substrate.
I prefer using the 1/2 thickness. It will cover more imperfections that mat be hidding on the sub floor, is thick enough to screw and plug, and most importantly, is thick enough to trim the edges of both the floor cut out and the hatch.
Never, Ever put in a hatch without adding solid teak to the hatch and the floor opening. Have seen " good " carpenters do this. Then when the hatch is removed or re-inserted.....the veneer chips !
When the ply is being laid in area that is prone to water, I west system the bottom by mixing west epoxy and adding about 10% laquer thinner. This makes the ecpoxy more water like and it actually soaks into the ply very well.
The topside , be it in a head or other areas gets about 8-10 coats of satin polyuerathane. I am not a fan of using epoxy first, then varnishing. I prefer satin finish and the poly because a satin finish will show less scratches from everyday use and is very easy to apply touch up coats.