Which would live longer? A fish out in the open air? Or a dog submerged in water?

A fish in the open air. The metabolic rate of the exothermic fish is much lower than that of the endothermic dog. Thus, oxygen utilization is much lower for the fish and gas exchange requirements are lower.

Many fishes travel across the land to reach a new body of water. As long as its gills are still wet, it is fine. Sometimes the common names reflect this (walking catfish, mud skippers), sometimes not (eels).