We discussed this when I did my cylinder testers course and the consensus of opinion was apart from being potentially more dangerous, that it is 'old hat' and a complete waste of time putting a cylinder in water to be filled.
Unless it is being cooled (which I have never seen), the water is going to be at more or less ambient temperature so being in water or air isn't going to make the slightest bit of difference.
You can hang out in shorts and a t-shirt in 50f all day. You can even work up a sweat and need to cool down. In 50f water you will be hypothermic in a short period of time.
When you are rejecting heat through convection, the mass of the fluid(air and water are both fluids) is quite important. Movement of the air or the water will greatly increase the movement of heat. What you are doing is employing the second law of thermodynamics. The molecules in the air or the water become more excited and the molecules in the tank become less excited. The water has much greater density of molecules available to accept the heat energy from the tank than the air does. Movement brings more molecules into contact with the surface of the tank and keeps the delta T of the molecules in contact with the tank at a higher level.
Greater deltaT (temperature differential) equals greater heat exchange
Greater density of the cooling fluid equals greater heat exchange
Greater movement of the fluid equals greater heat exchange
Longer dwell time equals greater heat exchange
If you fill the tank in a water bath but do it in 5 minutes and then pull it out then the benefit of the water will be limited by the low dwell time. If you do it in water that is room temperature then the effect will be greater than if you do it in water that is 90f but 90f water will still cool a 130 degree tank faster than 70f air. If you have a small volume of water and fast fill lots of tanks the water will get very warm. It has accepted a huge amount of heat but it's capacity to continue doing so is now diminished.
I disagree strongly with anyone that claims it isn't faster to cool a tank down in water than in air but the amount of benefit is dependent on the above factors.
I am ignoring radiant cooling in my description. It is a small factor with it's own set of variables but it does have a greater impact in the air cooled side of this equation. Actually because air cooling is SO ineffective, radiation does play a significant role in that version.
I apologize for the lecture. I'm not good at this.