Yes, a boat or some other means of water transport will be required to get to the area of the cache. At an hour or so either side of low tide this will be above the water level.
Once at the cache site you'll need stout footwear and perhaps some leather gloves. Do NOT attempt with bare feet! While placing, both TB and GSV got a scallop slice to the fingers so be warned.
The easiest spot to start from would be Ravensbourne - see Parking coordinates. It's only about 400m. to the cache from there.
As the main challenge is to get to the site, the picture was taken very close to the cache site.