DRG AN ILS TO RWY 35, THE APCH WAS NORMAL UNTIL THE ACFT WAS APRX 100' ABV THE DECISION HEIGHT, EXCEPT THERE WAS CONDENSATION ON THE WINDSHIELD. TOUCHDOWN WAS HARD. A WITNESS SAID THE ACFT LNDD APRX 1/3 OF THE WAY DOWN THE 5200' RWY. AFTER LNDG, WITH THE PROPS IN 'GROUND FINE,' BRAKING ACTION WAS INEFFECTIVE ON THE WET RWY. THE ACFT WAS NOT EQUIPPED WITH AN ANTI-SKID BRAKING SYS. SUBSEQUENTLY, THE ACFT CONTD OFF THE RWY & WENT DOWN AN EMBANKMENT INTO A FIELD.SCRUB MARKS ON THE RWY & REVERTED RUBBER ON THE TIRES INDICATED THE ACFT WAS HYDROPLANING. ACCORDING TO THE FLT MANUAL, 4500' (PLUS A 10% FACTOR FOR NO ANTI-SKID) WOULD HAVE BEEN REQUIRED TO STOP ON A DRY RWY IN THE PREVAILING WX CONDS, WHICH INCLUDED A SLIGHT TAILWIND. TWO PAX, ACTING AS FLT ATTENDANTS (F/A'S), ASSISTED THE OTHER PAX IN EVACUATING. AN INVESTIGATION REVEALED THE F/A'S WERE NOT PROPERLY TRAINED; THE PAX WERE NOT COMPLETELY BRIEFED BEFORE THE FLT; & THE PAX BRIEFING CARDS WERE INADEQUATE. THE ACFT WAS ON A BUSINESS FLT TO TRANSPORT AN ENTERTAINMENT TROUPE.