A BE-58 pilot declared an emergency and descended to a lower altitude while flying IFR and accumulating ice on his anti-icing equipped aircraft.

Narrative

I filed an IFR flight plan from ZZZ to ZZZ1; time XA10Z. My BE58 Baron is certified into known icing conditions. Weather for ZZZ1 at my ETA was FEW 600 FT; 8;900 FT BROKEN -- VFR. The route I filed with ATC was ZZZ VOR VXXXX ZZZ1 VOR VXXX ZZZ2 VOR VYY. There was an icing potential in the area of ZZZ2. Although a Piaggio Avante did report light rime ice at 7;000 FT around ZZZ2; forecast ceilings along my Flight Plan Route would have provided an escape to lower altitudes; out of visible moisture; and to the nearest airport should inflight icing become a problem. ATC cleared me as filed; so I proceeded with the flight. Prior to reaching the ZZZ1 VOR; ATC advised me of a change in routing. My new routing was to proceed to ZZZ1 VXXX ZZZZZ Intersection direct ZZZ1; maintain 7;000 FT. This routing brought me over the water; where no known ceilings were reported. After I received my amended clearance; I immediately switched on all my anti-ice equipment. After about 5-10 minutes on VXXX; I started accumulating light rime ice on my nacelles and the leading edge of my rear horizontal stabilizer. At the first sign of ice accretion; I activated my deicing wing boots to break any accumulated ice. After 10-20 minutes of making and shedding ice; I notified ATC that I was making ice and needed a lower altitude. ATC said; 'Sir; if you wish to declare an emergency; there are about 20 airports behind you.' Since I was 1/2 way from the shore and ZZZ1 on VXXX; I told ATC I wanted to declare an emergency and requested immediate vectors to ZZZ1 at a lower altitude. So they cleared me direct to ZZZ1 and told me that I could descend to 5;000 FT. I felt this to be the correct action since my wife and children were aboard the aircraft and my wife was growing increasingly anxious during this time. Upon reaching 5;000 FT; ATC queried me to find out the conditions of my flight and to see if I was still making ice. I was still making ice at 5;000 FT; so they descended me to 4;000 FT. At 4;000 FT; I was still making ice. So I asked for lower and they told me to descend and maintain 2;000 FT. at 2;000 FT; the ice was no longer a factor. As I approached the ZZZ1 area; the sky was brightening and the weather was clearing. I initially asked ATC for vectors to the ILS Runway 24 at ZZZ1. After hearing ATC talk to an air carrier indicating that they had a Baron with an emergency; I opted for the visual approach to Runway X as I did not want to hold up air traffic since the emergency no longer existed. I safely landed on Runway X at ZZZ1 without incident. Had I known that my routing was going to change; I might not have gone in the first place. At the time of the emergency; my wife was becoming very anxious in the cockpit being that we were flying with our 2 children. This only added to the urgency of the situation as well as to my own anxiety. The reality was that the icing was manageable. I did not lose any airspeed or experience any buffeting of the airframe. However; I didn't want to stay in the ice at 7;000 FT; over the water; in IMC for any longer than I had to since the stress in the cockpit started to become an issue. So after being prompted by ATC; I declared the emergency in order to descend and to get direct routing to ZZZ1 since that was where the clear weather was anyway. Thinking about the situation now; I feel I should have declined the routing that ATC gave; demanding a different routing along a known escape route. In the future; I will think twice about flying in icing weather with my family on board.