Oh, man that sucks. I frankly don't know of one made specifically for a marine application. What you could do is use a brass swing check valve. Your local hardware store will carry them. Be sure to get a size or two large than the hose size so the valve doesn't pose a restriction. I would also make sure your through hull fitting is brass. You do not want that fitting to break. All so, you could may braze the check valve right to the hull fitting for a cleaner installation. The check valve would work the best when it's in the vertical postion.