I think you mean "
Skulls and Shackles." Sorry a bit of a pet peeve of mine, anyhow to answer your question. AP's are written with 4 players in mind with point buys of 15. So if you run 6 players then keep in mind that they will probably blow through the encounters that are pre-written and you'll have to add a little more oomph to keep the CR's up to par with the party. The is even more difficult if you allow a higher point buy or rolls which can sometimes equate up into the 35-40 points if bought.
As far as advice with this particular AP, make sure to have your players read the free player's guide and come prepared with traits/feats/character builds that will help them on a ship (i.e. not smart to have a cavalier who is mounted on a horse and uses a lance to charge his enemies, smarter if they play a swashbuckler, etc.). If I was running it I'd require that one of their two traits was campaign specific from the Adventure Path, this gives them a tie-in to the start of the adventure and helps them out by also giving them something that's useful for them.