Actually, heat doesnt rise. Hot air rises which gives us the perception that heat rises, but most thermal transfer in a computer system is conduction, not convection.
The problem with having a large tank of water over the CPU is cooling it off, if you just have the heat going into the water the water will slowly heat up, it wont cool off nearly as quickly as the CPU will push heat into it so after an hour or two you would have a large tank of boiling water and an overheated CPU, thats why water cooling loops have radiators in them, to bring the temp of the water back down to room temperature.