Ram ventilation is why tuna (and more famously, many [but not most] sharks) die if they stop swimming. Other fish use buccal pumping, which is what fish are doing when they move their mouth. They're using their buccal cavity, that is, the inside of their mouth, to pull water in and push it over their gills. Ram ventilation is moving through the water with an open mouth to allow water to pass over the gills. Many fish actually use both when either is appropriate. The only other ways to oxygenate blood in water that I know of are external gills (think axolotl) and cutaneous respiration (skin breathing, think frogs).