DURING A NIGHT CFR PART 135 CARGO FLIGHT, THE CERTIFICATED COMMERCIAL HELICOPTER PILOT AND A CREW MEMBER RECEIVED COMPANY INSTRUCTIONS TO DIVERT FROM THE INTENDED ROUTE OF FLIGHT TO ANOTHER DESTINATION. THE WEATHER CONDITIONS IN THE AREA INCLUDED AREAS OF LOW CEILINGS AND REDUCED VISIBILITY DUE TO RAIN. THE PILOT ATTEMPTED TO CROSS A COASTAL BAY IN A WESTERLY DIRECTION IN AN AREA OF LIMITED VISUAL CUES AND DECIDED TO REVERSE HIS COURSE. THE PILOT CONTINUED SOUTH AND AGAIN ATTEMPTED TO CROSS THE BAY NEAR A BRIDGE THAT SPANNED THE BAY. A WITNESS OBSERVED THE HELICOPTER DESCENDING BELOW THE LEVEL OF THE BRIDGE WHERE VISIBILITY WAS ABOUT 1/4 MILE IN RAIN. THE HELICOPTER STRUCK THE WATER ABOUT 1/2 MILE FROM THE SHORE AND ABOUT 400 YARDS SOUTH OF THE BRIDGE AND WAS DESTROYED. THE PILOT HAD OBTAINED A WEATHER BRIEFING THAT INCLUDED AREAS OF MARGINAL VFR DUE TO RAIN AND FOG, THUNDERSTORMS, AND SEVERAL AIRMETS AND SIGMETS.