At 200' cross check with your radar altimeter. If they don't agree go-around and reset your altimeters. Do the after takeoff checklist, climb checklist, descent checklist and before landing checklist, and then, and only then, attempt the ditching.
The investigators will be all over you if you continue the approach with the wrong altimeter settings!
Repeat after me - procedure, SOP, procedure, SOP, procedure, SOP... it will save you.