Low tide would work best, but it can be reached on high tide. Do not attempt during stormy seas.
It will take at least 3 hours to do just this cache.
You will need to hike the St Blaize trail in order to get to the first waypoint.
From there, follow the wooden path down to the beach. Walk across the beach, over the rocks, untill you reach the last waypoint.
Once you get to the last waypoint, you will be standing in front of a large cave.
Walk 80 steps straight in. In front of you should be a large boulder covered with green moss. The cave splits here to left and right.
Turn 90 degrees right, and walk 20 steps.Turn 90 degrees left here, and walk 30 steps.
In front of you should be a large whitish rock. About 5 meters wide, and 3 meters high.
Just around the left corner of this rock.