This is a really interesting spot. At the published co-ordinates, there is some historic grafitti on a concrete barrier "WIPEOUT THE PIPEOUT". I call it historic, as it is barely readable, but it is there, and someone went to the effort of putting it there.
An interesting aroma at this spot confirms that this is a sewerage outflow point.
As do the signs warning you not to swim, or collect shellfish.
So, when you get to GZ, there is a warning sign.
The cache can be found at:
S41 21.ABC E 174 51.XYZ
where:
A = the number of words in the top line of the sign
B = the number of fish depicted on the sign
C = the number of letters in the first word on the second line of the sign
X = the number of shellfish depicted on the sign
Y = C
Z = the most common number in the telephone number depicted on the sign
checksum A + B + C + X + Y + Z = 20
The cache is a camo'd 200ml systema, with a log book only, please bring your own pen.
And breathe deeply at GZ for the final cache location, the air is much fresher here.