One small note: I would suppose you would want a rifle with as little recoil as possible for this application. Being not solidly seated and anchored but freely floating might make what is normally a bump or push to the shoulder into a sudden thrusting backwards. Also I think being immediately over the surface of the water will make the noise of the shot considerably louder because the surface would act like a wall containing and reflecting the blast along a plane very close to you. .243 may be the one for you here.
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