Boundary Island Mystery Cache

This is one of a series of puzzles on BOUNDARY ISLAND that might fill in some of the harder gaps in your Difficulty/Terrain grids. The difficulties range from 1.5* to 4* and all have a 5* terrain rating due to requiring some form of water transport access.
None are at the listed coordinates.

Boundary Island is a popular mooring spot for houseboats or other aquatic day trippers around the Mandurah estuary.

Pull up at the jetty - the parking coordinates - and enjoy a paddle, picnic lunch and maybe a few coldies. Fishing and crabbing is often productive in these waters, too.
Then why not start the treasure hunt!?
 

Grab a hire boat or canoe in Mandurah or BYO, take a picnic, good walking shoes, hat, water, mozzie spray and coordinates for the other caches here and make a day of it.