I have a low profile tunnel hull and the bilge outlet sits below the waterline.
Has anyone heard of a one way valve that I can plumb inline so water doesn't seep back down into the bilges?
Evan

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.

Is there room to put a loop in the bilge hose as well to help keep the water out?

There are two types of valves, sold at your local marine store for this. If they don't stock them, they can certainly order them. One is called a "flapper-valve", the other is called a "Strato-Flo valve". Both are brass/bronze. Both have female pipe threads that would call for a male pipe threaded brass/bronze bilge pump discharge thru-hull to mate up. All this, you might want to know, is a very compromised system at best. Your bilge pump discharge thru-hull being located below the water line is a big "no-no". Failure of these valves will quickly sink your boat by allowing water to free-flow through the bilge pump discharge line & into the boat. I would strongly suggest you install a bilge-pump discharge thru-hull in a new location farthur forward on the boat, which is well above the water-line, & run a new hose forward to it. This is the only safe & correct way. Ed

One other thing to keep in mind is that most bilge pumps are rated at little or no back pressure or lift. So trying to push water out of a below water line fitting would be less effecient.