Start with current quest gripes
Pretty simple, riddle clues need to have a minimum chest associated with them... The game has been out long enough that, most of the community cancel them now as it's too much time to sail to a single Island to solve a clue with a high chance of a dud chest. Knowing the riddles only have 1 chest on the island makes it hard to want to do it.
Same with the X maps, I cancel pretty much any quest that only has one X on the map, it's just not worth the time investment. Either have a higher drop rate of captains chests or above for single X maps, or at least make all maps have a minimum of 2 chests you can dig up.
Here's a solution:
What about at least put extra riddles or treasure maps that can be found ON THE ISLAND themselves... You'd still have to search for them. Either random bottles spawn on the shores, ontop of barrels, scribbled on rocks, anywhere really, that a player can find ON THAT ISLAND that contains clues, or treasure maps...
Hell, island clues could be even screenshot sketches of a particular part of the island. (Think Red Dead Redemption treasure hunting maps) Surely these could be scattered around each island.
Having this extra way of finding treasure will create a reason to stop at ANY island. The chests found in this way could even be considered a whole new chest to themselves. You could probably just do it for the larger island, cause the smaller ones will be too easy to exploit.
