THE PLT AND HIS PASSENGER (WIFE), WHO HAD BEEN SEPARATED FOR SOME TIME, DEPARTED ON A LOCAL FLT FROM WEEPING WATER, NE. ALTHOUGH NOT VERIFIED, THE PURPOSE OF THE FLT WAS BELIEVED TO BE FOR PERSONAL REASONS. LATER, SEVERAL WITNESSES OBSERVEDTHE ACFT FLYING LOW & SLOW OVER THE PLATTE RIVER. A WITNESS NEAR ASHLAND, NE, RPRTD THE ACFT WAS SO LOW THAT THE PLT INITIATED A CLIMB TO CROSS OVER A BRIDGE. WITNESSES LAST SAW THE ACFT FLYING UP THE MEANDERING RIVER (UPSTREAM) AT LOW ALTITUDE. SUBSEQUENTLY, IT WAS FOUND WHERE IT HAD CRASHED IN 3 TO 4 FT OF WATER, ABOUT 30 YDS FROM THE SHORE. THE ACFT CRASHED IN A STEEP NOSE DOWN ATTITUDE, ON A DOWNSTREAM HEADING. THE NOSE OF THE ACFT TOOK THE BRUNT OF THE IMPACT, BUT THERE WAS ALSO DAMAGE TO THE FORWARD COCKPIT, WINGS, WING STRUTS & LANDING GEAR. THE EMPENNAGE & AFT FUSELAGE WERE UNDAMAGED. NO PREIMPACT PART FAILURE OR MALFUNCTION OF THE ACFT WAS FOUND. TOXICOLOGY CHECKS SHOWED THE PLT HAD A URINE/ALCOHOL LVL OF 0.108% & A BLOOD/ALCOHOL LVL OF 0.058%.