There isn't a one way valve I've ever found that has a low enough pressure open point to work with a bilge pump. I've dealt with the problem you describe before by simply routing the bilge hose up inside the gunnel or transom as high as possible (like a hump) to keep the water from backflowing into the bilge from outside. You can the the ty wraps the the glue plate on them and use a little epoxy to stick them right to the underside of the top of the transom or deck and hold the hose up in that manner. At least this has worked for me in the past a couple times.