Therefore, if the coil entered at the top, it would have the maximum differential temperature over the tank water, giving up heat to exit cooler.
... isn't that backwards? The largest differential will surely be when the hot flow meets the coldest water - ie entering at the bottom of the coil.
Not that I'm arguing - just wondering if you can clarify your thoughts a bit further. Are you talking about a completely cold tank to start with, for example, or one with stratified layers at different temperatures already present?