Yes fish are easy to drown I've done it countless times. Take a fish put it on a stringer, line, or your hands, start motoring in a t 5 or 10 knots- when you get in the fish will all be dead. The water moving through their gills is to fast to extract the oxygen needed to survive. It happens all the time, that's why boats have live wells, it'd be easier to stick them on a stringer and use the space for other things. But you'll only bring back dead fish- garanteed
They would suffocate. Think of it this way, if a person got put in a glass bubble full of breathable air, eventually the air would run out of oxygen, and the person would suffocate. If a fish got put in a glass bubble full of oxygen rich water, eventually the water would run out of oxygen and the fish would suffocate.