According to those opinions that only one who flies over an ocean has to say hagomel, why does one not say hagomel when flying within the USA (where one often takes off over an ocean)?
The Shulchan Aruch in 219:1 (based on Brachot 54b, which is based on Tehillim 107) lays out 4 categories of individuals who are required to make Birchat Ha'Gomel, one category is "יורדי ים" - literally: 'descender to the sea', colloquially: 'seafarers'.
The Kitzur Shulchan Aruch in 61:1 explains that this halacha includes
Opinions differ regarding whether one should say this blessing on all flights or just those crossing the ocean/desert (see here for a litany of opinions) and one should certainly consult with his/her local Rabbi to establish the local custom/halachic ruling.
It seems that the debate surrounds 2 issues, the first being halachic in nature and the second practical:
Accordingly, according to an opinion that states that one should say Birchat Ha'Gomel when crossing a sea on an airplane, the danger threshold of that situation may be limited to only crossing over entirely and not just passing over briefly (see US Airways 1549).
Those who say that you say hagomel when flying over the ocean, mean that you cross an ocean. (That is actually the language used in your reference question) If you leave JFK but head to Florida, you aren't 'crossing the ocean' in the way that it is meant. However, if you fly to Mexico,(over the gulf of mexico) you would be.
The question is, is the flight a substitute for traveling over land, or a substitute for taking a boat?
(One should also bench gomel when crossing a desert, not just an ocean)