Hi! I had a pirate themed birthday party for my DS (now 12) when he turned 4... this was way before POC movie but we were inspired by the POC ride.... we didn't have a boat and my guy was much younger but this was one of the all time best boy parties...
We had treasure map invitations.
We played pin the tail on the donkey-.. except it was a treasure chest on a map and you had to get it on the X.
There may have been another game, I don't remember. I wanted to do a scavenger hunt but decided that 4 was too young.
The hit of the party was that we, (ok... my mom), made hats for everyone... and eyepatches... and swords. The hats were folded butcher paper (white). The eyepatches were black felt sewn to 2 black ribbons. The swords were cut out of cardboard and then the blade was spraypainted silver and the handle was left cardboard. I had all sort of stickers, paint, markers, glue, glitter and little plastic jewels and gold coins for them to decorate their swords and hats. The little girls loved that- most of the boys could care less.
However, girls and boys ran around and played swordfight for most of the party. I also handed out necklaces and coins as "loot". The cake was also a treasure map.. very easy to do since I am cake challenged.
I think that your guys would have great fun with a scavenger hunt- with or with out a boat. The swords were a huge hit and the kids (including friends and neighbors) talked about them for years.
Have fun! I think I'll do this party again my little pirate is old enough!