What distinguishes all self-propelled submersible vessels from surface ships?

The distinguishing feature of all self-propelled submersible vessels compared to surface ships is their capability of remaining submerged. This ability enables submarines and similar vessels to operate undetected beneath the water's surface, providing strategic advantages in military operations, such as stealth and the ability to launch torpedoes or conduct surveillance without being easily targeted.

Surface ships lack this ability, as they remain on the water's surface and are more vulnerable to detection and attacks. The capability to submerge not only affects combat strategies but also influences the design and engineering of submersible vessels, ensuring they can withstand the pressure of the ocean depths and operate effectively while submerged.

While other options may relate to specific mission capabilities—such as engaging in land assaults or operations in shallow waters—they do not universally apply to all self-propelled submersible vessels or serve as a defining characteristic when compared to surface ships. The essential feature is their submerged capability, which fundamentally sets them apart.