This is an easy Jackanism chest to get. When you start the Shipwreck City level it's right behind you so just spin round and get it!
Just like the other chests, you'll need to have 3 shrunken heads in order to unlock the chest, once you have them press "RS" and you'll open the chest.
For a full guide to everything you need to collect in the game Crimzen Dragon has made an excellent guide here;
http://bit.ly/fv8aPF