tristanc said..
Is it worth bringing a foilboard to yorkies knob, or is it too shallow?
Aside from the jokers...

Yes mate. Yorkeys and Trinity Beach are good for foiling, but don't go near the river banks as they tend to be shallow and full of beginners (or crocs!) anyway. Both need high tide for foiling as long as you can launch in a small beach area above the tide mark. Or walk out 200mt to waist high water at least.