Seems mostly okay to me, as a non-native. Three comments:

(1) This story is usually translated with the diminutive Русалочка -- not only because Andersen called her "little," but also because she's cute, young, and lovable, whereas the non-diminutive русалка often implies a creature closer to the Greek sirens (who were beautiful, but not at all lovable, and in fact rather deadly). Possibly the main difference between the Greek sirens and the Russian русалки is that the former lived in saltwater and the latter lived in freshwater rivers or lakes. So it's kind like the difference between lobsters and crayfish -- but they both have painful pinches! Thus, if you're talking about a sweet-natured mermaid, go with the diminutive русалочка. However, it would probably be okay to describe Ursula the Sea Witch from the Disney movie as "a type of русалка," because of that word's more sinister connotations.

(2) "вода же синей, как синего василька" -- You should ask native Russians, but I agree with your instinct to use a comparative construction in the Russian ("bluer than a blue cornflower"). However, with this construction, you don't need the как -- you can just use the comparative adjective + the genitive singular the thing you're comparing it to. Thus, "Вода синее синего василька". Similarly, "яснее ясного кристалл".

(3) Finally: "поверхносты и" -- remember that all of those feminine -ost nouns (гласность, etc.) end in a SOFT т, and thus the genitive singular ends in -ти, not -ты.