If the bowl is below the waterline, yes
Water will rise in the intake line just as it can in the discharge line, and when the head is below the waterline, water rising in either line can overflow the bowl. The vented loop in the intake protects your boat from sinking in the event someone leaves the "dry/flush" valve in the "flush" position, or the valve fails--which is less likely, but without a loop you're betting your boat that it won't. It goes between the pump and bowl, at least a foot above the waterline at any angle of heel. Do NOT put it between the thru-hull and the pump...if you do, the pump can't prime.
