Gases dissolve in water at cold temperatures but solids dissolve in water at warm temperatures? Is this because of the geometrical properties of Carbon, Oxygen, and Hydrogen? Also, is it better to dissolve carbon dioxide in the water and kill coral or dissolve it in the air which leads to global warming?
When most gasses dissolve into a solution it's an exothermic process. The gas molecules give up energy to do so. An increase in temperature leads to an increase in kinetic energy. Higher kinetic energy causes the gas molecules to break their intermolecular bonds and escape from solution. Note I say solution and not water. It's not just water that has this property.
Your third question isn't really a physics question and so I'll leave it to the ethicists, biologists, etc.
Experimentally it has been found that dissolution of a gas in a liquid is an exothermic process.
Gas + Liquid -> Gas/Liquid + heat (∆H = -ve) ....(i)
So according to Le Chatelier's principle, if I decrease the temperature, the reaction will move in that direction which doesn't let this change (decrement in this case) take place. For that to happen the temperature has to be increased which can happen if the reaction (i) proceeds towards the forward direction. Therefore the gas dissolution will increase on decreasing the temperature.
An interesting fact: cold water supports aquatic life because if the temperature is low then the dissolution of gases will increase i.e, the dissolution of oxygen will increase and the fish will get a high supply of oxygen.
On the other hand, if the temperature is increased the dissolution of gas will decrease and there will be less supply of oxygen.
So, fish prefer to live in cold water than in warm water.