I've shot Shi Shi a couple times before, it's a cool location.
Another variable on the fins shot is the sand. Weather/waves move sand in and out and can cover the fins to a large degree. Simple answer is that I'd plan on shooting with whatever you get when you're there. Weather is variable, it's a long drive from anywhere, and it's a 4 mile hike each way (1 mile of nice trail, 1 mile in mud & water, and 2 miles on the beach). Plan on making a couple visits if you can.
Check out Marc Adamus's image from there. He's got a great one, as do Chip Phillips and Kevin McNeal. If you trust the EXIF on Kevin's shot, his sunburst image was taken on July 31st so you won't be able to get that shot. I think the stuff with good water in the fins may be taken in the winter when storms scour out the sand... but that's no guarantee either. I've shot in January and the fins were fairly well covered then too.