BEFORE DEPG ON THE ACDNT FLT, THE PLT OBTAINED A WX BRIEFING & WAS ADVISED OF RAIN SHOWERS ALONG HIS ENTIRE ROUTE. ALSO, HE WAS WARNED OF THUNDERSTORM ACTIVITY THROUGHOUT ALL OF CALIFORNIA. HE FILED A VFR FLT PLAN & TOOK OFF. SHORTLY AFTER TAKEOFF, HE ACTIVATED THE FLT PLAN, THEN THERE WAS NO FURTHER COMMUNICATION WITH THE ACFT. ABOUT 1 HR LATER, THE ACFT WAS OBSERVED FLYING AT LOW ALTITUDE. AS THE ACFT APCHD A SIGN, IT WAS OBSERVED TO ENTER A CLIMB & DISAPPEAR IN A LOW CLOUD LAYER. SHORTLY THEREAFTER, IT WAS OBSERVED BY ANOTHER WITNESS IN A DIVE AS IT DESCENDED BELOW THE CLOUDS. MOMENTS LATER, THE ACFT CRASHED IN A LAKE. AN EXAM OF THE WRECKAGE REVEALED THE ACFT HAD IMPACTED THE WATER WITH VERY HIGH VELOCITY. NO PREIMPACT PART FAILURE/MALFUNCTION OF THE ACFT WAS FOUND. THE NON-INSTRUMENT RATED PILOT HAD ONLY ABOUT 3 HRS OF INSTRUMENT FLT TIME. WITNESSES RPRTD THUNDERSTORMS & LIGHTNING IN THE AREA; THEY ESTIMATED THERE WAS ABOUT A 500 FT OBSCURATION WITH 1/2 MI VISIBILITY, BLOWING RAIN & WIND FROM 135 DEG AT 15 GUSTING 20 KTS.