Possible Clues:

• Navigable in winter, say
• Like navigable Arctic waters
• Lacking frost
• Fierce, wild extreme climate without frost
• Unfrozen, as harbors
• (Of water) not frozen
• Warming Arctic may become so fierce, spoiling one of Earth's extremes
• Unlike a rink, without frozen water
• Without frost
• Most ports are unlike, say, scotch on the rocks