This is about the anime characters. For the monster itself, see Zilla.
In Episode 16, the Zillas try to sink Captain Horn's ship in order to kill the Searchers as they cross the ocean. Even though they are very large, they are very cowardly and turn back after the ship fires a few cannonballs at them.
During their attack, the Zillas use their massive heads and tails to ram the ship and try to push it over.