EDIT: Don't buy more than 2 upgrades in shield in order to keep your ship small and your cannonfire concentrated... Otherwise itll go all over the place
Here's a strategy that works quite well.
Spend all your gold on crew members, and your excess money on cannon amounts. Crew = boarding power/reload speed. with 160 pirates (maxed out upgrade), you can board any ship and win. You can beat a 40 sailor ship and take only about 5 casualties. After maxing out crew, try to max out cannon amount, and put some points into cannon damage when you find your cannons not doing as well as you want. Don't forget to put some gold into ship armour when you find yourself dying from cannonfire. Also, a trick to stop ships from boarding you is to wait until they are on the other side of the screen, then sail upwards, and the other ship will move up, and will redo their whole 'side-to-side' advancement.
THERES A GUIDE TO THE WHOLE THING - http://www.youtube.com/watch?v=RSsJ19sy3JI
IF THAT DOESN'T WORK TRY THIS ONE ITS GUARANTEED NO MATTER WHAT THE FIRST ONE IS -
http://www.youtube.com/watch?v=03BaTyXTMNA
3-1 get the key bottom left, then run to top right and keep jumping at the wooden thing till u get over. grab the key up there then push the wooden ball so the wooden stairs swing over. get the key and go for gold
3-2 - jump up and to the left and push the wooden block over so you dont need to get the red key.
theres a 27-life walkthrough video here
http://www.youtube.com/watch?v=RSsJ19sy3JI
if that doesnt work try this :P
http://www.youtube.com/watch?v=03BaTyXTMNA
3-1 get the key bottom left, then run to top right and keep jumping at the wooden thing till u get over. grab the key up there then push the wooden ball so the wooden stairs swing over. get the key and go for gold
3-2 - jump up and to the left and push the wooden block over so you dont need to get the red key.
This works for real: complete the game in one day on the upgrade menu keep pressing tab until you get to the repair ship button. then press enter until it repairs completely you will go into minus money! Copy/ paste this if it worked for you! Keep it living!! Please note that if you spam enter it stops working and you have to do another day as it goes into kong chat and doesnt come out. The repair stops working, u press tab and end up in the chat like I just said, WOOT THIS GLITCH IS AWESOME! It really works witouth cheat engine
 "BoomsticK"){kind=icon}
 "Spewer"){kind=icon}
 "Mud and Blood 2"){kind=icon}
 "I Love Traffic"){kind=icon}
 "Treasure of Cutlass Reef"){kind=icon}
 "SAS: Zombie Assault"){kind=icon}
 "Super Crazy Guitar Maniac Deluxe 2"){kind=icon}
 "Super Crazy Guitar Maniac Deluxe 3"){kind=icon}
 "Don't Look Back"){kind=icon}
 "Mastermind: World Conquerer"){kind=icon}
 "Monsters' Den"){kind=icon}
 "Particles"){kind=icon}
 "SeppuKuties"){kind=icon}
 "Shore Siege!"){kind=icon}