wrex, in the past the Sandcastle Competition has been held on the sand at the bottom of the wooden pier opposite the Imperial Hotel.
It makes sense because the Imperial Hotel is the sponsor of the annual event.
Therefore, it follows that the organiser will always check the situation regarding the tide before setting a date and time for the event.
I am not an expert in building sandcastles but I would imagine it will be best to do with dry sand which suggests it will be better to arrange when the tide is coming in rather then just after it has gone out.