Semi-submersible naval vessel
A low-profile/semi-submersible naval vessel is a hybrid warship, that combines the properties of a surface ship and submarine by using water ballast to partially immerse and minimize its above-waterline profile, thereby improving its stealth characteristics when in hostile waters. The USS Monitor was an antecedent to such craft with its low-profile deck and gun turret.
Source: Wikipedia — Semi-submersible naval vessel (CC BY-SA 4.0)