Are you suggesting to something specifically like they were disposed of in the water or beneath the sand?Yes, there was the extended boating regatta at the time
as well as the Test Cricket which he was an avid fan of,
and there are statements from school friends of his siblings saying they discussed what they did over that Christmas period and that a trip to Adelaide was mentioned.
then you have Percy's own statement to he his old friend (who was police officer) that he was in fact at the beach when the BCs disappeared
Oh and the hardness of the soil is irrelevant when at sea...