what scheme did jim have in mind

From the text:

As for the scheme I had in my head, it was not a bad one in itself. I was to go down the sandy spit that divides the anchorage on the east from the open sea, find the white rock I had observed last evening, and ascertain whether it was there or not that Ben Gunn had hidden his boat, a thing quite worth doing, as I still believe. But as I was certain I should not be allowed to leave the enclosure, my only plan was to take French leave and slip out when nobody was watching, and that was so bad a way of doing it as made the thing itself wrong. But I was only a boy, and I had made my mind up. ch 22