What was the primary reason for the Nautilus's unique semi-submarine design?

The Nautilus was designed as a semi-submarine vessel to protect its vital components from enemy fire during combat^[1]. Its unique design ensured that if it went into action, only a small oval platform carrying machine guns and the conning-tower would be visible above the water^[1].

The rest of the vessel, including the hull, would be submerged ten to twenty feet underwater, placing it beyond the reach of conventional shots^[1]. This allowed the Nautilus to operate with a significant defensive advantage.