DRG LIFT-OFF FROM AN OIL PLATFORM IN THE GULF OF MEXICO, THE HELICOPTER BGN A SLOW UNCOMMANDED LEFT TURN. THE PLT TRIED TO FLY IT BY LOWERING THE NOSE & INCREASING COLLECTIVE. AFTER 2 TURNS, THE HELICOPTER SETTLED & IMPACTED THE WATER IN A LEFT BANK, NOSE DOWN ATTITUDE. THE AUTO-PLT WAS NOT DISCONNECTED, NOR WERE THE FLOATS INFLATED. NO PREIMPACT MECH FAILURE WAS FOUND. INTERVIEWS REVEALED THE HELICOPTER WAS NOT STABILIZED IN A HOVER BEFORE INITIATING A TAKEOFF. THERE WAS A PREVIOUS WRITE-UP WHICH STATED 'A/C REQUIRES MORE RIGHT PEDAL THAN USUAL IN HOVER AND FORWARD FLIGHT.' IN RESPONSETO THIS WRITE-UP, A MECHANIC HAD ADJUSTED A TUBE WHICH RAN REARWARD FROM THE PEDAL ASSEMBLY TO THE LOWER PORTION OF THE AUTO-PILOT SYSTEM. THE MAINTENANCE MANUAL DESCRIBED THIS TUBE AS BEING ONE OF THE 'SEMI-FIXED' RODS. REFERRING TO THESE RODS, THE MANUAL CAUTIONED, 'THEY ARE ADJUSTED ONCE FOR ALL WHEN THEY ARE FITTED TO THE AIRCRAFT. THEY MUST NOT BE ADJUSTED SUBSEQUENTLY.'