"A folyón úszik a görög hajó, vagy a tengeren?"

Translation: Is the Greek ship sailing on the river or on the sea?

[MidknightPhoenix]

can úszik be used to mean a boat sailing like this?

[vvsey]

Yes, Hungarian uses "úszik" for ships and boats and such.

[Krisbaudi]

Why is swimming wrong? Are motorboats also sailing?

[Judit294350]

Boats do not "swim" in English. Motorboats motor (and the Hungarian verb I think would still be úszik.