Regarding the practice of double clamping hoses raw water hoses found below the water line - the usual drill is that hoses be secured with two (2) all stainless steel hose clamps. ABYC only recommends that exhaust and stern tubes be double clamped to prevent accidental flooding. If the hose barb is of sufficient length to land a second hose clamp – then double clamping raw water hoses is strongly advised. Unfortunately, some hose barbs are too short to accommodate two clamps. If the second clamp lands just past the end of the barb there's a chance that it might pull the attached hose partially off the fitting - so be careful about double clamping.