THE PLT REPORTED THAT DURING A DESCENT TO 2100 FT, INTERMEDIATE LEVEL OFFS WERE MADE AT 5000, 4000, & 3000 FT. HE STATED THAT AT 2200 FT, CARB HEAT WAS REMOVED & POWER WAS APPLIED FOR THE LEVEL OFF. HOWEVER, WHEN THE THROTTLE WAS ADVANCED, THE ENG BEGAN RUNNING ROUGH & WOULD NOT SUSTAIN LEVEL FLT. THE PLT STATED THAT ALL COMBINATIONS OF POWER & CARB HEAT WERE TRIED, BUT PROVIDED NO CHANGE. THE POWER LOSS OCCURRED OVER A DENSE FOREST, SO THE PLT ELECTED TO MAKE A FORCED LANDING IN A LAKE. THE ACFT SANK IN 14 FT OF WATER. IT WAS RECOVERED, BUT NO PREIMPACT PART FAILURE OR MALFUNCTION WAS VERIFIED. THE CARB HEAT CABLE WAS LOOSE, BUT THERE WAS IMPACT DAMAGE IN THAT AREA. THE TEMP & DEW POINT WERE 74 & 68 DEGS RESPECTIVELY. ACCORDING TO ICING PROBABILITY CHARTS, CARB ICING WOULD HAVE BEEN POSSIBLE IF THE CARB HEAT HAD NOT BEEN USED OR HAD NOT FUNCTIONED.