Misleading sentences in your question
fishes can breathe in water (at least for few-hours)
Fishes can breathe for much longer than a few hours under water as they spend their whole life underwater.
Animals-with-lungs (such as most terrestrial mammals)
All mammals, whether terrestrial (like a cow) or not (like a dolphin) have lungs. While all mammals have lungs, most fish (but not all) have gills.
Lungs vs Gills
To understand the answer, you need to understand two key organs involved in gathering oxygen from the environment: gills and lungs.
Lungs are organs that allow the transfer of oxygen into the blood when inhaling air, while gills are organs that allow the transfer of oxygen into blood when inhaling water. Lungs don't do their job properly in water and gills don't do their job properly in the air. You should have a look at the following wikipedia entries to further your understanding of these organs:
does fishes require much-much less-amount of oxygen than lunged terrestrial-animals, though they stay in physical activities including predation?
I am not sure I understand the meaning of this sentence but anyway.
Making generalization about fish metabolic rate can only be misleading as there are an immense diversity of fishes. For example some fishes are endotherm (see here to understand the definition of this term) while some are not and this will vastly affect the metabolic rate.
Although eventually too advanced for the OP, Chabot et al. (2016) is an interesting reading to get an general overview of the Metabolic rate in fishes.