I translated this as "surface water", and got it wrong. The official translation is "surface of water", which I find awkward in English. Does this phrase mean "the surface of the water"?
"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.
So could you also use it in the context of 'the surface of the lake'/'powierzchnia jeziora'?
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.