Do not depend upon a check valve to prevent inflow of water via siphon when the pump is not running. Put in a vented loop well above the water line (Forespar makes some decent quality Marelon vented loops).
If you want, you can put a check valve in the exhaust
hose near the pump to try and keep the water that is in the hose prior to the vented loop (which would otherwise flow back into the bilge
area you're pumping out) in the hose.
Note that a check valve does restrict the outflow of water, so adding a check valve does reduce to some degree the volume of water the pump can push.