A HELI PLT IS UNABLE TO MAINTAIN VFR AND DECLARES AN EMER UNTIL VFR AND LANDS AT GLS.

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AT APPROX XA00 LCL; I LANDED ON AN OFFSHORE PLATFORM AND SHUT DOWN. THERE WERE ISOLATED RAIN SHOWERS IN THE AREA; BUT THE WINDS WERE LIGHT AND I OBSERVED NO LIGHTNING. AFTER ABOUT 1 HR; THE SHOWERS WERE GETTING CLOSER; MORE PREVALENT; AND THE WINDS INCREASING. I ADVISED THE CUSTOMER WE SHOULD GO. AT XB35 LCL; I TOOK OFF TOWARD GALVESTON. ABOUT 5 MI INTO THE FLT; I RAN INTO RAIN SHOWERS. I TRIED TO RETURN TO THE PLATFORM BUT COULDN'T; NOR COULD I SEE THE BEACH (WHICH WAS VISIBLE WHEN I UNTIED THE ACFT). I TRIED TO MAKE ANOTHER PLATFORM A FELLOW PLT HAD TOLD ME WAS IN THE CLR; BUT AGAIN RAN INTO RAIN AND REDUCED VISIBILITY. EVENTUALLY I WAS UNABLE TO MAINTAIN VFR AND SURFACE REF WITH THE WATER. I WENT ON INSTS; CLBED TO 1000 FT MSL; CONTACTED HOUSTON APCH; DECLARED AN EMER AND ASKED FOR VECTORS TO SOMEWHERE TO GET ME DOWN. GALVESTON WX WAS 9 NM WITH FEW SCATTERED AT 1100 FT. I DSNDED TO 800 FT AND FLEW THE ASSIGNED HEADING (010 DEGS) TOWARD GALVESTON. AT ABOUT 5 MI OUT; I SAW THE BEACH AND CONTINUED TO GALVESTON. I REMAINED WITH APCH UNTIL I HAD THE FIELD IN SIGHT; THEN CLOSED WITH APCH ON LNDG.