PRIOR TO DEPARTURE, THE CAPTAIN REVIEWED AVAILABLE WX REPORTS & WAS AWARE OF THE WX CONDITIONS, INCLUDING CURRENT SIGMETS. HE DECIDED TO FLY A COURSE THAT WAS PARALLEL TO A FRONTAL SYS UNTIL HE COULD GET TO AN AREA WHERE HE COULD CONTINUE TO HIS DESTINATION. AFTER DEPARTING, HE LEFT THE SEAT BELT SIGN ON, BUT DID NOT BRIEF THE PASSENGERS TO EXPECT TURBULENCE NOR RESTRICT THE FLT ATTENDANTS TO THEIR SEATS, SINCE HE DID NOT EXPECT TO ENCOUNTER TURBULENCE. WHILE CRUISING AT 17,000 FT MSL, HE USED HIS RADAR AS WELL AS ATC ADVISORIES TO IDENTIFY ADVERSE WX. AFTER TURNING TO AVOID AN AREA OF HEAVY PRECIPITATION, THE ACFT ENCOUNTERED TURBULENCE. AT THAT TIME, 3 FLT ATTENDANTS & 1 PASSENGER WERE WALKING IN THE AISLE OF THE ACFT. ONE PASSENGER WAS SERIOUSLY INJURED. ONE FLT ATTENDANT & 6 PASSENGERS RECEIVED MINOR INJURIES. THE ACFT WAS THEN FLOWN BACK TO TAMPA, FL WHERE IT WAS LANDED WITHOUT FURTHER INCIDENT.