First of all, We do have about 23 miles of coastline on the Atlantic Ocean which we will also call 'the sea'. We also have rivers, brooks, ponds and lakes; none of these are the sea, although it's possible to occasion a creature coming from the sea into them (salmon made a habit of it). Nevertheless, creatures native to the streams, ponds and lakes are not deemed 'sea' creatures.
Secondly, the term hornpout is, at least originally, applied to our small variety of fresh-water catfish. None of our ponds and lakes are likened to the Dead Sea, nor any sea, so the streams feeding or draining from them are not salt-water.
Thirdly, hornpout are bottom-dwellers that don't appreciate the cooler waters of our faster moving, rocky-bedded streams. If you're going out on the river, it's not for hornpout; try casting your line into a fish pond around sunset for better results. Also be careful removing a hornpout from your line -- their horns (or stingers) can really puncture.