The cache is not at the posted coordinates! No need to swim or take your boat to Heron Island.
You will need to complete my nearby Adventure Lab entitled "Take a Walk on Bill's Peninsula" to get the final coordinates.
The actual cache you will be looking for is one of those magnetic hide-a-keys we all love. Inside I have placed a few of my signature pathtags for those who collect them.
I hope you enjoy this geocache and the aforementioned Adventure Lab!