A bilge pump discharge is usually thru the hull and high above the waterline. This is done for two reasons.
1) So the Capt. can physically see the water stream and verify that his/her bilge pump is actually working.
2) To prevent sea water from entering into the bilge by keeping the discharge line well above the waterline.
If you put a check valve in your bilge pump it will become a potential point for clogging if debris should get caught in it. Additionally, it will lessen the efficiency of your bilge pump by making a restriction in your line which will both lessen the water flow and shorten your pump motor's life by placing a strain on it.
No way would I ever put a check valve in my bilge pump discharge line. In fact, to absolutely get the best performance out of my pumps I never use the white plastic corrugated hose you usually see with them. I go to Home Depot and buy the smooth nylon reinforced clear vinyl hose of the proper size. First, it's a lot stronger and second, it's smooth on the inside so there is almost no static pressure resistance to the flow of the water. (And it will last longer than your boat probably)
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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