What makes a power plant suitable for a warship?

What makes a power plant good for a ship?

It needs adequate power and power density, practical maintenance, and the ability to withstand marine conditions.

The answer evaluates a ship power plant by more than headline horsepower. It must fit sufficient output into the available volume, remain maintainable at sea, and tolerate a corrosive saltwater environment. The short answer provides a compact engineering checklist for comparing propulsion systems across ship types.