Treasure hunt, deep sea
I have no idea of what limits you have, or game has to implement fully new things, if there needs to be an existing plugin, or maybe some plugin can be adapted to this, but got this idea, so thought i'd share it anyway.
Would be fun if there was more to do in the deep sea, so my idea would be that you find treasure maps in containers on the main island, like the pet signal. And they would show a buried treasure in the deep sea islands, either someway on the map, or in the same way as pet signal gives direction and distance.
It would require forexample you to use the metal detector on correct spot for a chest to pop up.
Treasure could consist of a lil bounty (200-400) medium (600-1200) to a large (1500-2500) of scrap. Or whatever works best.
It could increase the traffic (meet people boating) and the motivation to build boats and sail over to the deep sea more often.