[bertil]

That certainly limits their universality, but you can imagine a network of fast trains when there’s ground and boats, or something faster on sea, when there’s none, to substitute to flying.

Ekranoplanes use the ground effect, which requires to be close to a flat surface, indeed, but the larger the place, the further you can be. Something the size of a 747 could be a couple dozen meters above the waves and fly smoothly above moderately rough sea.