I got off work early yesterday and headed down to Third Beach to see what it looks like at low tide (which is when our ceremony will be). I think it'll turn out really nice, large parts of the beach are empty at low tide because the sunbathers all stay above the high water mark and not too many people go swimming. So, since the parts of the beach most likely to be empty are the wet sand parts, you will probably be most comfortable in flip flops or bare feet.
I'd heard some horror stories about ceremonies at low tide being graced by seagulls and errant seaweed all over the place and sand lice and funny smells, but I didn't detect any of that.
There's one particular spot which I think would work really nicely. If you're familiar with Third Beach, it's right at the very very north end, where the sand ends and rocks begin. There's a patch of sand (I think big enough for 40 people?) with rocks to the north, water to the west, and a huge boulder to the east. It's semi-private and really only accessible at low tide.
Afternoon low tide on Aug. 6 will be 2.3 m at 3:37 pm. As it happens, Thursday's afternoon low tide is also 2.3 m, so I think I'll make another visit just to see what it looks like then. It was out to 1.1 when I went yesterday, so not the best indicator of how much sand will be above water when I'm trying to get married.