Re keeping the banner on ..
Many years ago, a colleague was involved with Navy gunnery target towing in a cabin class twin. During some low level manoeuvring, the target dipped in the sea .... have you heard the expression .. "carrier landing" ? Fortunately, the boys were at a reasonable cruise speed and managed to jettison the line before they stalled .... but, it appears from the tale, the incident did get their attention ..
Suggest you really would not want to face the high level of interest associated with flying a nil engine operating water glide landing when the banner dips in the water with the tug aircraft (depending on the length of the tow line) a considerable number of feet above the water .... the stall (spin ?) may prove fatal regardless of anything else ?