"The surface of the water" is fine in English. Somebody or something can be "below, at, above the surface of the water", for example.
But "surface water" means something else - water that is found on the surface of the ground or planet, for example a pond or river.
Hmmm. I think my first thought if I heard "powierzchnia jeziora" would be "surface area of the lake". Technically "surface area" = "pole powierzchni", but in everyday language "powierzchnia" is enough. So I'd say "powierzchnia wody" to make it unambigous, but "powierzchnia jeziora" would work as well if the context was clear enough.