you pressure test the water cooling system same way your head was tested,
you get someone qualified to do it, obviously they need the car to be with them

remove spark plugs and if you get a drop in pressure,
the water may be getting into the bores,
how to tell if there is water in the bores after a pressure test,
poke something in the holes where the spark plugs go

the symptoms are not a cracked head, you tried 2 turbo`s and have identical results
both turbo`s could be shot, but without a pressure test,
its still guessing at many things,
you could also try cranking over with no turbo fitted and push a sheet of cardboard over the manifold studs and bolt up to catch any oil or water particles
and examine after,