If it is just dull, a compound like 3Ms may bring it back. If it is oxidized (white hazy milky looking finish) then the only way to get rid of it is by wetsanding. Once you get it back use BTS (bow to stern) to protect it from the UV rays that couse this and keep the shine. good luck.
The key is to get the oxidation off first then coat with whatever product trips your trigger. BBS, BTS, Wax, etc. The degree of oxidation will determine what you need to do to get rid of it. Start small with a rubbing compound and work up to wet sanding because that is a chore!! Once you have it gone, the goal is to keep it gone.
DO YOU USE 600 GRIT TO WET SAND AND WOULD WET SANDING REMOVE THE CLEAR COAT.I'VE BEEN USING THE RUBBING COMPOUND BY MEGUIARS MOST OF THE AFTERNOON TODAY AND STILL HAVE SOME OF THAT MILKY LOOKING FINISH WHEN THE SUN CATCHES IT JUST RIGHT.I'M USING THE RUBBING COMPOUND , THEN MEGUIARS POLISHING COMPOUND AND THEN MEGUIARS WAX.
There is a lighter wetsanding paper of like 1200 grit I think. I hate to send you to another site but there is a forum on BBC (BassBoatCentral) that is nothing but restoration. go have a look.