Kaiou means "sea king" but the maru part is kind of untranslatable. Suffice to say, it's a common suffix added on the ships. There are many stories about why, but one of the most common ones is that the word maru, meaning "circle" is a good omen allowing the ship to return (or circle back) to the harbor where it left on the often perilous journey. It's added to a lot of other objects now as a sign of affection as well.