when do i unlock more naval missions in assassins creed 3?

yes more will show up. you just need to progress through the main storyline to get them to pop up.

Just go to a harbormaster and select the icons on the dotted paths to activate the missions

Hope this helps

You get it after series 6. pass to any of the harbormasters placed in Boston and that i've got self belief huge apple besides. once you spot different countries, pass down the map to Mexico and you will see an exclamation mark, indicating the mayan venture initiate.