As the name 'Bilge' keel implies they should be fitted along or just under the bilge, lower part of the hull.
Fitting at the waterline will have little or no effect as they are intended to work together.
If the roll is far enough to lift one out of the water the effect will thus be diminished.
'Waterline' can also vary according to the load!
Naval ships that I have been involved with (patrol boats up to LPDs and light aircraft carriers) all have bilge keels approx 1/3 to half the hull length fitted amidships, just below the round bilge angled out at ~40° to the horizontal.
On small fishing boats / patrol boats I have seen them fitted under the bilge almost vertically, thus also giving support to stop 'em falling over when beached at low tide😉
Same applies to hard chine hulls, fit below the chine line at the mid section.
Hope this helps, cheers, Doug 😎