Years ago as a fresh-out-of-college kid I managed a small restaurant that had no safe, and no way to store the till's contents overnight. I cut the bottom out of a large black olives tin and tucked the cash bag into it, and then set it in among the other black olives cans on the shelf above head height.
The restaurant was broken in to one night, the place ransacked as the thief looked for the money. But the money was not found and remained safely on the shelf.
A key to the success of the ruse with a hide-in-plain-sight gimmick is making sure it's an obvious low-value item, and it helps if there are more than one real one with the hideout tucked into the group like it belongs.
I don't know how many canned goods you might have on board, but sail bags you likely have in abundance.
I like the false waste holding tank idea too.
What about the plastic fender/bumpers? I think one could be crafted to be a hide-safe, and nested among the real ones... just don't forget and dangle it over the side!