There is a mission where there is a treasure on an island and I don't remember how, what to do to get some little dude to get off boat, go and retrieve the treasure How/what do you do please

in order to retrieve the treasure, you need to establish a settlement on the island where the treasure is located, and have the treasure location within your market area coverage i.e. you have to build a market place further in the island if necessary to gain possession of the treasure, as soon as you have it within your coverage area you'll see a blue man walking out of your market building and going to retrieve the treasure, cheers :)