Hydroships

"Hydroship" is just a word used to differentiate from ships used in space and the water. A hydroship is the water kind. That's pretty much it.
Hydroships are most commonly large boats (think like the USS Arizona or the Abraham Lincoln, or some other battleship, carrier, destroyer, etc.), but anything that is made to function as a watercraft can count. Rowboats technically qualify as hydroships. Some hydroships, such as those made by the Arthromedon, don't even touch the water.