A B733 CREW; LNDG ON A WET RWY AT SMF; WERE UNABLE TO NEGOTIATE THE EXIT; SLIDING OFF THE TXWY.

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EVENT: WHILE EXITING THE RWY TURNING ONTO THE TXWY THE ACFT SLID OFF THE PAVEMENT APPROX 30 FT INTO THE GRASS AND MUD. SIT: THE FO FLEW A STABILIZED ILS APCH TO RWY 16L IN SMF. WE LANDED ABOUT 1500 FT DOWN THE RWY ON SPD AND ON CTRLINE. BECAUSE OF THE BAD WX AND RWY CONDITIONS; I DECIDED TO BE CONSERVATIVE AND ROLL OUT THE LNDG ALL THE WAY TO THE END OF THE RWY RATHER THAN TAKE THE HIGHSPD TURNOFF. UPON SLOWING TO TAXI SPD; I CAME ON THE BRAKES WITH THE FO AND TOOK CTL OF THE AIRPLANE. I PUSHED THE PWR UP SLIGHTLY TO KEEP US ROLLING DOWN THE RWY. UPON REACHING THE END OF THE RWY I SLOWED THE AIRPLANE TO TURN OFF ON TXWY D11. WHILE IN THE TURN; THE ACFT BEGAN TO SLIDE. THE ACFT SLID OFF THE TXWY AND INTO THE GRASS AND MUD. THE AIRPLANE STOPPED APPROX 30 FT OFF THE TXWY. THERE WERE NO INJURIES. I BELIEVE THE CAUSE WAS BAD WX YIELDING A VERY SLIPPERY TXWY. CORRECTIVE ACTION: THE BIGGEST CORRECTIVE ACTION IS TO ADVISE ALL PLTS TO BE ULTRA CONSERVATIVE (WHICH I WAS DOING BY GOING ALL THE WAY TO THE END OF THE RWY). WHEN THE RWY/TXWY IS WET; HYDROPLANING IS MUCH EASIER THAN ONE THINKS. SINCE THE INCIDENT; I HAVE LEARNED THAT OTHER ACFT HAVE SLID/HYDROPLANED AT THE SAME SPOT. I BELIEVE A COMMENT ON THE RELEASE OR THE FORM WOULD BE BENEFICIAL TO ENHANCE SAFETY. SINCE THE INCIDENT; I HAVE ALSO LEARNED THE FO SAID HE WAS 'A LITTLE UNCOMFORTABLE' WITH OUR TAXI SPD. HOWEVER; HE NEVER SPOKE UP TO RELAY HIS FEELINGS. THIS FACT NEEDS TO BE EMPHASIZED IN INITIAL TRAINING SO NEW FO'S CAN BE A BETTER PART OF THE COCKPIT TEAM. THEY NEED TO VOICE THEIR OPINIONS IF THEY SEE SOMETHING THEY DON'T LIKE EVEN IF THEY ARE NEW!