chlsbrns, it won't work. I'm not talking about "coil sizes" here, for the moment, but .... just anything you can "tow" or "drag" underwater like you're thinking. Perhaps for finding concentrations of metal-in-general (like if you're trying to locate shipwreck site or something). But no, not for individual fumble fingers targets, like rings, etc....
I learned the hard way when this happened: There was a muddy estuary tide-finger to the ocean, where we were helping the sherrifs look for a gun (that had supposedly been thrown into this water-way, by the fleeing trigger-man). So we picked a minus tide day, assuming we would walk right out on dry land. However, it was sticky gooey mud, and we would immediately sink up to our knees! Doh. So we devised another ingenious method: Wait till high tide, and take a flat bottom skiff or canoe out there. Then simply drift with the breeze, while having the coil lowered into the ~ 3 or 4 ft. deep water. Sounds genius, right? Wrong. Well, sure, we'd get "beeps". But then there is simply no way to re-swing over the object, since you are blind to the bottom (cloudy dark water). And as you try to swing left and ring to center (to get an idea of the size, if it was the object we were looking for or not), was impossible. Because the boat is subtly drifting, so you have no reference point of swinging, re-scanning, etc.... And praytell, even if you COULD get a signal, and COULD pinpoint it (ableit blindly which I'm saying won't work), then how did you plan to retreive it ?
Thus no, sledding, towing, dragging, etc.... is going to be an exercise in futility, when it comes to individual objects. No one gets a single "beep" and digs. We ALL criss-cross, move subtly up and back to size out, etc... And those type things can not be done from above water, unless you're standing there, feet planted firm and stationary. Can't be done from over a boat. Unless, as I say, you're merely looking for the presence of metal in general (shipwrecks, etc..).
