PLEASE make sure you secure the lid fully before you leave it!
Hidden over the edge of the park you will find an old concrete base. Hop down the side and hope that the tide is out enough to be able to access the cache. Otherwise you may need to wade out. Once you are on the concrete base, you will need to find a hollow Mangrove tree. It is not too far away from the edge of the platform.
I dont actually know what this once was but I can only presume it used to be a beacon to warn the ships of the edge of the land.
The old metal pole has either been cut down or has rusted away. Either way there is now a foot of rust.