The oceans arent flat and smooth all of the time so as soon as it gets rough things start to fall apart, with the added corrosiveness of salt water these crafts would have to get cleaned fairly often and would destroy engines, plowing into the ocean at 500 kts would be a bad time, passenger experience might also become an issue because flying over the ocean at 500 kts would be rather terrifying.
[Obligatory Mustard video](https://www.youtube.com/watch?v=yVdH_dYlVB8)
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