[rolph]

solubility of gases in water is a general phenomenon, yes water is solvent for a lot of things, and that exacerbates the situation, more gases, mean partial solubility rather than the whole solubility, so that would mean even less oxygen dissolved in a volume of water.

you need to update your model of physical chemistry, there are a number of issues to your detriment.

as temperature increases, gas solubility decreases across the board, solvent caging of water is inhibited at higher temperatures.

the issue is not only oxygen. water must be solvent to ammonia, and CO2 to an extent compatible with physiology.

it wont matter how much O2 is dissolved if you cant excrete your waste products.

CO2 dissolved in water at quantity will convert partially to carbonic acid, lowering the pH of the water, and dissolving Ca++ Carbonate this means its harder to make a shell, and what shell you have is dissolving at an increased rate.