You don't "have" to add a thru-hull ducer, although it will make it way easier to keep nice clean readings up to speed. I would decide to add one or not to now though. That way if you do decide to go with two ducers you can put your SI ducer a little higher up on the transom to protect it from debris while under way. I do some river fishing, and I keep my fingers crossed at each outing on the river. May have to move the SI ducer up and add a 2d just because of this......