I did that simply because the islands wee large, and it felt silly to have only one stack of coins per island.
One thing I’ve done in the past is have different coins in different areas of an island. A ship can only load coins from where it docks. This is easier when you have islands with multiple beaches to dock at, but you could also let crew leave the ship. Perhaps a crew can move L per turn on the island, and can carry one coin at a time back to the beach.