Assassin's Creed IV : Black Flag Walkthrough

I stopped playing it for a while in the middle of the game, now I lost track of the walkthrough series I was watching in Youtube. My objective is "tail the gunboat" and my story completion percentage is 25%

Could u kindly tell me which walkthrough episode I've to watch?

Thanks

Note - I was watching this series

why don't do you put the name of the mission on youtube and watch the walkthrough rather then looking for that mission on the link you provided.