Im a boat builder
At the boat yard i work at we only use 3m. This for a $3M riv or a 6ft tender
Best results is to use machine polish with a foam head not wool head.
keep a spay bottle with water handy light coat of water just stops the polish from drying up. Dry polish is what makes the swirl marks you see on cars, once you have finished a area go back over it with a clean dry rag
the job will come up spiffy
NOTE
do not use an aggressive polish unless you are trying to remove scratches (you will see the polish marks) If your just trying to make you boat shine fine polish is the way to go.
Most polishes are the same but 3m is made for marine and has a better finish than car polish on fiberglass
Cheers