Found on f.e. the "Cycladic Archipalego" maps, "Sporades Islands", and the random map "Island Stronghold". Petra already checks on land, so it makes sense to check on water as well.
`gatherTreasure()` is moved to entityExtend.js so it can be used on non-worker entities.
In a future step, the naval manager could assign a specific merchant ship for each sea `accessIndex`, and send it to search for treasure; in this diff, naval traders will simply check on occasion if there is any nearby treasure.